At least voting for a 3rd party one can be said to be contributing to a cause closest to one's ideal, but being favorable to anyone who the mainstream doesn't approve of just seems like flawed logic.
Anti-establishmentarianism can bring together people of different political persuasions who all want to dismantle parts of the current system. What comes after that is a new battle, and that one may end up fought along more traditionally ideological lines.
Corbyn for instance, very much left wing, opposed EU because of all the globalist bankers and buerocrats in Brussels controlling the society, cheap labor bringing down wages.
If you're a bit older or have better knowledge, then trumps policies are far more similar to someone like Sanders if they really wanted to team up. (I'm not talking about his speech and the way of saying things)
I actually have supported with many actions of Antifa / BLM (probably wouldn't guess that from my post history), including the protests at Pelosi's and McConnell's houses, and the disruption of the globalist meetings you mention. I don't agree with them in their destruction of property, especially that of small mom-and-pop businesses, or even big business if it affects small businesses (by raising insurance rates for example). And obviously, I disagree with anything actually threatening lives.
If Hitler invaded hell I would make at least a favourable reference to the devil in the House of Commons. -- Winston Churchill
Yes, I'd rather have a social democrat that rejects Stalin than a fiscal conservative that embraced him.
Unlike the various 'what-ifs' of policy issues, anyone can examine the actual record of Stalin and his policies. And, in my opinion, anyone who finds him a source of inspiration, regardless of their stated policy preferences, has a severe lack of judgement.
This seems absurd. If your political reasoning puts you in a situation where Trump and Sanders are both options for you then your system is broken - they are antithetical to each other in style, philosophy, character and most importantly policy. Based on your logic, if both parties rejected Stalin then you'd be in favor of him.