I do not believe that Donald Trump is a Russian plant. I do not believe he has the intelligence to work undercover like that. He may have agreements with the Russians, but I do not believe that the Russians would use somebody as untrustworthy as Donald Trump if they needed a plant in the US government.
This seems too ludicrous to contemplate, although I do suppose they could have recruited him early in life and offered him untold rewards for his cooperation. However, what is the payoff here. I think Trump is a dangerous jerk, but I think the Russians may not own him as much as they think they do.
No, Trump is not likely a Russian plant in the sense that Russia did not specifically place him in a position of power for their own gain. However, they do seem to be taking full advantage of the opportunity to use Trump now that he is in a position of power. Funny thing is, the Republicans seem to be doing the same thing.
Russia wanted him to win but Russia didn't orchestrate his entire campaign. They helped him win but I think they would have worked to help any Republican candidate win. Europe can warn Trump all they want, their words are just going to fall on deaf ears. Trump has his own agenda that involves benefiting his family and friends and that will be his top priority.