I suspect what happened is that the hacking groups gave Wikileaks the dump via a cut-out. It's entirely possible that cut-out isn't aware how the data was obtained.
I don't think Assange is a Russian agent (even though he receives money from RT etc). I think he has his own motives. At the time this was more anti-Clinton that pro-Trump specifically.
More recently his Tweets have become more supportive of Trump personally (although interestingly not really his agendas necessarily). My uncharitable suspicion is that he's hoping for a presidential pardon.
this same evidence would have been "page 6" news and would have been rather quickly forgotten but for a few "sore losers".
Of course. There's plenty of similar evidence that the Russians also supported the Green party and Sanders campaigns in various forms. No one talks about them because they ended up not mattering.