1.) Agreed, although I would generalize that more as a refusal to do what even such people can see is morally right. (Paul’s own way of putting it involves those who do and who do not know the Torah.)
2.) Agreed. No salvation apart from God, thus also not apart from God the Son (nor apart from God the Holy Spirit for that matter), whether apart from God’s choice to save sinners from sin, or apart from sinners coming to cooperate with God (in all three Persons).
Non-trinitarians won’t agree with the trinitarian points of that, of course, but the basic points still stand: no salvation apart from God, from God’s Messiah, and from God’s and/or the Messiah’s holy spirit (whatever those mean).
3.) Agreed, but I would go on to argue from the contexts, including the referential ones in vv.13-15 and nearby, that God’s first and foremost evangelist is God Himself (or the Messiah for non-trinitarians). The feet of those on the mountain announcing good news are YHWH’s feet after all; non-Christian rabbis recognized them as the feet of the Messiah, too/instead.
To the question then: “Besides Jesus going and preaching to the spirits in prison, which appears to be a past event, is there any biblical mention of an ONGOING witness to Jesus Christ in hell, pointing people to the way, the truth, and the life?”
(I’d dispute that the preaching to spirits in prison was only a one-time past event, but let that aside. That you’re willing to consider it as post-mortem evangelism at all is more than most people can do!)
A lot of Christians (whether trinitarian or not, whether universalist or not), myself included, regard the Holy Spirit and His (or its) work in our heart, as the chief method by which God brings sinners to repent of our sins and come to the Son. Consequently, a lot of us Christian universalists, myself included, regard this as happening post-mortem, too – including in hell, where we’re purgatorial universalists.
The only scriptural place I can recall offhand that (I’d argue) shows the Holy Spirit (and the redeemed church as the Bride) evangelizing impenitent sinners to stop fondling their sins and accept Jesus Christ as their savior, is in the second half of the final chapter of RevJohn. And Christ is still there, too, pictured (poetically) as the river of life flowing out of the never-closed gates, with Christ thus also cooperating to bring sinners to Christ.
That’s a very specific level of imagery, although not without dispute of course (not least because of the poetics involved). I would argue that “the fire the eonain” in Gehenna, prepared for the devil and his angels, which Jesus talks about salting everyone and being the best of things and which leads to being at peace with one other, is the Holy Spirit operating on people in hell. A fairly large Christian tradition (among the Eastern Orthodox for example), even where it isn’t universalistic, agrees that the eonian fire is God Himself (whether the Spirit or the Son or both) in inescapable presence to impenitent sinners. But I can’t say it explicitly shows the fire leading people to accept Christ there.
On the other hand, there’s a significantly large amount of scriptural testimony, mostly in the OT, to the effect that YHWH Himself (and/or His King Messiah) will be instrumental in a massive, and massively successful, evangelical outreach after YHWH comes to make Himself blatantly known and knowable by the whole world. This isn’t usually disputed, although sometimes not accounted for; what’s disputed is whether any of this testimony counts as post-mortem evangelism. I think some of it does, but the poetic nature of the material can count both ways: suggestive imprisonment language can count as being actually resurrected from the dead and into repentance and reconciliation with God and our fellow creatures; but on the other hand, what sounds like blatant resurrection language might be a poetic description for a fate less than death from which the person recovers. I will however say that, in my experience, the reason most commentators (even non-Christian Jewish ones) think the OT doesn’t have much if anything to say about bodily resurrection and/or any kind of life after death at all (whether saved or lost), is because commentators are discounting such evidence – mainly because, if true, it would be evidence not only for bodily resurrection but also for post-mortem salvation. A systematic denial of the latter would necessarily require denying concurrent evidence of the former where they fit together; it would even count as evidence that the apparent bodily resurrection must refer to something else! Be that as it may, it’s a major study challenge. 
I think I can fairly say that language specifically describing ONGOING evangelism specifically to ACCEPT CHRIST, is thin at best. Usually I myself would argue it’s an inferential conclusion from various testimonies being put together. For example, this is the goal, God says He will definitely reach the goal, the goal necessarily involves X, there are various partial indications of X in scriptural testimony, therefore through deduction and through inductive support, I should and do conclude X happens.
So for example, if I’m right about Jesus testifying in John 5 that the purpose of raising those to judgment who are dishonoring the Son and the Father is so that all may be honoring the Son and the Father, where all includes them, too (which the righteous who lead many to salvation and who keep on being righteous will understand, even though the wicked won’t understand and so will keep on being wicked at least for a while), then the Son’s judgment must be leading them as an evangelical method to accept Jesus as the way, the truth, and the life, and so to honor the Son and the Father (and the Spirit) and so to come out of judgment and into eonian life. That the judgment is evangelical is a deduction from key data; that the judgment involves evangelizing those raised to olam abhorrence is supported by inductive data coming from the chapter of Daniel which Jesus refs there in talking about what’s happening.
Or again, John 6:44 involves people given to the Son by the Father being saved by being (rather explicitly in the Greek) “dragged” to Him: a topic directly related to them being resurrected on the final Day. Relatedly, all that the Father gives Him shall come to Him and shall not be cast out (v.36), nor shall the Son lose any of the all who have been given to Him by the Father. (v.39) Jesus doesn’t talk about repentance there, but in the very next verse (v.45) He directly refs the prophecy from Jeremiah 31:34 that all people from the least to the greatest shall come to YHWH to be taught by YHWH, “for I will forgive their injustice and their sin I will remember no more.”
So the topic is not about Jesus raising people who will never be given to Him, nor about raising already-saved people who are the only ones being given to Him, but about raising people who have not come to Him yet: but they will, and will be saved. That portion of Jer 31 doesn’t seem to reference the repentance and salvation being post-mortem; John 6 doesn’t seem to reference the repentance of those being raised, although it does reference repentance (if a bit obliquely) in the verb for being dragged to God. It’s only when both sides are put together that post-mortem repentance pops out, and for that matter total salvation of sinners eventually.
But then again, someone could turn around and argue from non-salvation having been solidly established elsewhere (they think), that John 6 shows Jer 31 must be talking about post-mortem action so can’t be talking about actual repentance and salvation; and someone could similarly argue from thinking non-salvation has already been solidly established elsewhere that Jer 31 shows John 6 must be talking about actual repentance and salvation and so Jesus can’t be talking about a post-mortem event after a literal resurrection from the dead (thus also that the “all” must be hyperbole for some, or refer to all kinds, etc.)
(Jeremiah 31 is also where God, having slain rebel Ephraim for his sins, comforts righteous Rachel that He has not forgotten Ephraim and will surely restore him once Ephraim learns his lesson and repents of his sins. God promises that He will accomplish this in a riddle where He shall be doing a new thing involving a woman encompassing a man. Matthew regarded this chapter as a prophecy of the virgin birth of Christ, and cites it as having been at least partially fulfilled in the deaths of the innocent children murdered by Herod; but it wasn’t innocent children who had been slain by God originally, so there is a much more important fulfillment on the way.)
But we’re trinitarians anyway, because we think that’s what the various data pieces add up to.
A perfect cure for the 