Well, that’s good, because JKH and Keith are directly contradicting each other on a pretty crucial point to your rebuttal attempt! You noticed that, right? 
So, which one do you prefer to go with there? Since they can’t both be correct. I’m going to guess you actually go with JKH on this, since Keith’s position requires a mere argument from silence. Although you’re pretty impressed by mere arguments from silence despite other evidence to the contrary, so you might be against JKH and in favor of Keith. Hard to say. (Or is it that it doesn’t matter to you that they can’t both be correct, so long as they’re contradicting each other against my position? Any stick is good enough etc.?)
Anyone else see where they’re contradicting each other? One or the other explanation could be plausible, but they cannot both be equally plausible.
Of course, I talked about the scenes from Rev 7 and Rev 15 both, with an extensive comparison to what’s going on in the final chapters of RevJohn, back when I wrote that extensive commentary on what’s going on in the final chapters of RevJohn. But you didn’t want to discuss those factors then. Apparently because I was the one who brought them up. No point discussing them when I do that; only when a non-universalist does it, hm?
Anyway, leaving aside (as an exercise for the reader) where they signally contradict each other: notably they actually agree with me almost everywhere, including what {ek} means. What they don’t comment on (at least in the brief paragraphs you quoted) is that their application of {ek} makes no sense when applied to the nearby comparative verse I mentioned. Remember what that was?
But meanwhile, a list of agreements between what I wrote and what JHK wrote:
• The group at Rev 15 are the same people as in Rev 7:14. (Although I would say they’re included in the much larger group from Rev 7. But the same principles apply either way.)
• The group at Rev 15 is described as “those who had been victorious over the beast…” (That’s one way to translate it, and J will be agreeing with the more particular way in a minute.)
• The word for victorious (incidentally, a plural noun “victors”, not the adjective “victorious”) is {nikao} and means “to be a victor, [to] conquer, to prevail”.
• {Nikao} is used with the preposition {ek} three times, one for each of the areas of victory–the beast, his image, and his mark. The three-fold reptition emphasizes the element of victory and deliverance. (I would say it emphasizes something else, too, but he probably wouldn’t disagree that it also emphasizes what they were delivered from. We would disagree on what it means to be delivered from that sin, I guess. 
)
• Here, {nikao} carries the idea of deliverance. (I wouldn’t disagree, but as I noted the term is also used in RevJohn in regard to personal choices to reject sin.)
• Because of their victory in Christ, they were delivered from the beast, from his image and from his mark. (What we disagree on is what this means.)
• These people, who are certainly believers in this scene, will find themselves living in the sphere of the beast’s power and under great pressure to worship him, his image, and to wear his mark, even to the point of death for refusing to do so.
• They will come out victorious from it all. (Sooner or later.)
The only thing we disagree about is something that happens not to be mentioned anywhere in or near Rev 15: 2-4, namely that these people refused to take the mark of the beast and are the martyred dead saints of the Trib. JHK doesn’t mention the evidence that this vision is looking forward far beyond the rest of the material afterward in Rev 15; nor does he mention the relevant connection to the Song of Moses, or even the connection to what John reports them singing directly.
But I sure did! 
(To be fair, he might have mentioned them other than where you quoted. But if so, you should have quoted more to try to answer me.)
Agreements with Keith:
• The Beast’s downfall only comes with the return of Christ, when he will be cast into the lake of fire.
• Rather weirdly, Keith and I actually agree that they don’t prevail over the Beast before then! (But I can’t figure out from the couple of paragraphs you selected from him, why he would say that, if the crew at Rev 15 are supposed to be saints martyred for not taking the mark of the beast. This position makes total sense under my theory; it makes no sense to take this position under his, so far as I can tell. But neither do I think Keith’s total attempt would be hurt by correcting this to something more coherent with this rest of his theory.)
• These saints are literally victors “from” him. (I actually agree with that more “literally” than he does! 
Where we don’t agree is that the Rev 15 saints are the same crew as back in most of Rev 14. More on that later.)
• {Ek} is used three times; means “out of, from, away from”. Therefore, a better rendering of 15:2 would be victorious “from the beast, from his image, and from the number of his name.” (Yes, he and I and the NASB center column are all on agreement on that!)
• Paradoxical as it may seem, “Christian” victory is often achieved by apparent defeat. We overcome by losing. (Not only do I agree, I’m taking that paradox farther than he does!)
• Those who overcome the Beast are not those who worship him, but those who worship God. (Similarly, those who overcome sin are not only those who never sin, but those who repent of their sins and… wait, that fits my concept better than his! It also much better fits the language and contexts of both Rev 15 and Rev 7. Including compared to Rev 14. More on that later, too.)
• Death is the way to life and to victory. (A good thing Christ shepherds the kings of the earth and their minions with the rod of iron then, isn’t it!? Wait, no, Keith doesn’t think being shepherded by Christ is the way to life and to victory, when it’s those people. Hm.)
• Christ overcame the world by dying to it, and so do we.
• In God’s kingdom it is those who would save their lives who lose them, and those who would lose their lives who save them.
• The saints of Rev 15 hold harps of God.
• Only four groups are mentioned as having harps in RevJohn.
(We disagree that the silence about harps in Rev 7 indicates harps are not given to all the martyred dead, although I can easily allow that that might be the case. We do however clearly agree that the group at Rev 15 is not the same group as Rev 14, although they’re on the same team! Which is weird because earlier it looks like Keith would at least include the Rev 14 group among Rev 15, insofar as the 144 thousand sealed would be very likely to largely overlap the martyrs who died early rather than take the mark of the beast. The difference is that I have an explicit reason in both texts to contrast the two groups, and you didn’t report one from him so far. 
)
• The harps contribute greatly to the heavenly harmony of the chorus that the redeemed offer to God. (Although between us, I’m clearly the one more emphasizing the notion of harmony of the redeemed before God! I might agree that the harps represent a privileged position before God’s throne, too. Certainly not a problem for my position in the least!–or else Christ’s moral to the parable of the 100th sheep is all wrong.)
So actually, your two sources and I agree very extensively, A. The difference is that I gave detailed reasons for my position, whereas they’re kind of just asserting their positions. (But to be fair, a couple of paragraphs don’t give room for much detailed reasoning.)
Among that detailed reasoning, was a boatload of evidence that the group at Rev 7 and at Rev 15 (where Rev 15 is included in Rev 7) are presented as being former sinners and idolaters who have been redeemed and brought home to Christ. They’re presented quite a bit differently than the group at Rev 14, who though doubtless also redeemed sinners, have some special qualities. They haven’t polluted themselves with women but are celibate. They faithful to be following the Lambkin and are a firstfruit of the harvest to God and the Lambkin. In their mouth falsehood was not found. And they are flawless.
By the way, anyone who accepts RevJohn canonicity and yet wants to read “eonian” to always mean never-ceasing and always-ongoing, will have to confess that the evangel being brought to those who sit on the earth and to every nation and tribe and language and people is also always never ceasing and always ongoing: because it’s expressly called the “eonian evangel” in Rev 14:6! A Calvinist might be able to get away with allowing this, by reading the unrestricted scope of verse 6 as really meaning only a restricted scope. But the Arminian expressly has to claim that this evangel isn’t the kind of ongoing that they insist is true for some punishment from God, such as that of those who take the mark of the beast in verses 10-11, and/or at 14-20. Otherwise, they’d be universalists! Insert irony here. Also of note is that the eonian evangel is delivered in this particular vision as so: “Be ye afraid of God and give glory to Him, for the hour of His judging has come; and worship the Maker of the heaven and the earth and the sea and the springs of water!” So the point of the judging is, in fact, to lead people to repentance. Does it work? The foretelling vision of 15:2-4 would indicate, by context, yes it does eventually succeed. But first, the events of the main narrative of the vision will take place.
At any rate, sauce for the goose is sauce for the gander here. If eonian is to be read as an adjective meaning ‘from the heart of God’ (or something like that), and not as merely meaning ongoing continuance (which eonian life certainly does not merely mean!), then the argument against universalism from appeal to that term and its application falls to ruin. But if eonian is only or primarily supposed to mean a never-ending continuation, then Arminians at least ought to be technical universalists on the testimony of the EONIAN EVANGEL of RevJohn! Which certainly fits the interpretation of the final chapters, too, as continuing the evangel past the point of the lake of fire judgment.
But, couldn’t the group at Rev 14 be just the same group as at Rev 15?–the 144K sealed who never took the mark of the beast?
While the Rev 14 group might be tacitly included in the Rev 15 group (other things being equal–which I have argued extensively elsewhere they are not), they cannot be simply the same group. Because the Rev 14 group sings a song that no one is able to learn except them (14:3); whereas in direct contrast the group at Rev 15 is singing the Song of Moses (which plenty of people, including Moses, were able to learn, and can still learn if they look up Deut 32!) and also a related song which John learned and (at least partially) reported in his Revelation! (15:3-4) That goes for the vastly huge group (which no human could count) singing back at Rev 7, too.
As for correspondences between Rev 7, Rev 15 and the final chapters, it would be tedious to repeat what I’ve already written extensively on. So go read that instead.
Besides, this will help consolidate it topically.