Some thoughts to the topic:
This what happened at the the siege of Jerusalem:
The death rate among the besieged increased. Soon, the Kidron valley and the Valley of Hinnom were filled with corpses. One defector told Titus that their number was estimated at 115,880. Desperate people tried to leave Jerusalem. When they had succeeded in passing their own lines and had not been killed by Roman patrols, they reached the palisade. Here they surrendered: as prisoners, they were at last entitled to some bread. Some of them ate so much, that they could not stomach it and died. In that case, their oedemaous bodies were cut open by the Syrian and Arab warders, who knew that some of these people had swallowed coins before they started their ill fated expedition. Titus refrained from punishing these violators when he discovered that there were too many. One of the defectors was the famous teacher Yohanan ben Zakkai, who escaped in a coffin and saved his life by predicting Titus that he, too, would be an emperor.
http://www.livius.org/articles/concept/roman-jewish-wars/roman-jewish-wars-4/?
That Gehenna was used as a garbage dump lacks historical evidence and relies on the explanation of a 13th century Jew. That Gehenna became the name of hell in the aftermath of this desctruction would not explain its prior use, especially that of James:
James 3:6:
and the tongue [is] fire, the world of unrighteousness; the tongue is set in our members, the defiler of the whole body, and which sets fire to the course of nature, and is set on fire of hell [Gehenna].
Does this imply that Gehenna was actually a fiery place at the time of Christ? I suppose the child sacrifices where too long ago to be present in the mind of the people from Jerusalem, so what was James referring to if Gehenna was not used as a garbage dump? Or was James simply referring to what Jesus had taught, how likely is it that actually Jesus was the first to employ Gehenna as the name of future punishment?
Concerning Jewish thought at the time of Christ, I have not yet read any extra biblical Jewish text that features remedial punishment in the afterlife, is there any source other than the Talmud? However Plato wrote that some are banished to Tartarus for a year and then might escape if their wickedness is not incurable, maybe the Jews got the idea from him.