Votes could be affected by preventing people from voting as well as by changing votes after the fact. The known facts point to either or both as plausible, assuming there was an opportunity.

You're certainly not alone in your doubts. Many people have assiduously been not assuming Russians had anything to do with tampering with the election every step of the way. Guccifer 2.0 was supposed to be a random Romanian hacker and yet it is now confirmed he was a member of the Russian military team attacking the US.

It's not helpful to dismiss stuff as a conspiracy theory these days in a general sort of way. The reason is that disbelieving one "conspiracy theory" requires you to believe in another, larger one. If you don't believe in the indictment, you are necessarily positing a massive conspiracy to frame Russia. There's no apparent simple, comforting explanation that doesn't involve large conspiracies.

Are there other nations that we have proof that they tried to tamper with that election? They of course need to prove it before deciding, but it seems to me that it is prudent to suspect the Russian state at this point.