1: No, we don't. "Everybody knows" is a dangerous expression, which really means "there is no proof, but someone with an agenda beats this into everybody's heads that they start thinking that they know". We can suspect. I would say Russia has a few advisors and is probably sharing intelligence with the leadership of DNR and LNR. However, all the claims of the "troops" are ludicrous exactly because they would have been so easy to prove by the adversary - the USA - who has all its satellites and grounds intel trained on this spot, looking for Russian troops and failing to find them for already almost a year.

PS, EDIT: There is one thing that I find fascinating. Is it so difficult to believe that a multi-million population can produce enough warriors, who are capable of holding off an aggressor? They are fighting for their homes, their families, their land. Unlike the attackers, they are highly motivated. They have access to the Ukrainian military hardware - both what was present in the military bases of Donetsk and Lugansk oblast's and what they captured from the invading forces. And finally, they have military command, who trained in the Soviet Army, many having the battle experience from Soviet-time hot-spots. They don't need Russian help (except when it comes to food and medicines). Just like Soviet Union didn't need help to beat the Germans out of its land.

2: Exactly. I said it before. Nemcov was hardly a problem for Putin while alive. His "opposition", having discredited itself by plundering Russia in favour of the Americans during the 90s, hardly ad 0.5% of support. While dead, Nemcov has suddenly become a much bigger problem. One can try demonise Putin in many ways, but you can't call him stupid to do this... A stupid politician wouldn't have survived in the turbulent post-90s Russia and wouldn't have managed to bring back order and restore some of the destroyed economy.