To be clear I was not a Bernie supporter either(since you mention lobbying the electoral system), I just used the MJ example because it was easy and at hand. I do not support Trump either.

By voting for someone I support, I am not voting anyone else into office, that is up to their supporters.

That makes sense emotionally, but in reality in FPTP you just enabled the even worse other party. If you want your voice fairly represented, you'll need other voting mechanisms and non-partisan districting mechanisms. Your wish is legitimate, but unfortunately the current system limits your useful actions.

There's no mathematical reason why the two winning parties in FPTP have to be the same two parties forever. It is the media coverage that makes it so, and concerted voting for a single third party could make that party part of a new duopoly.

Sure, you're right about that. In fact, we may be in an era where that might be possible at the moment (probably in the next election though). However, when a third party is actually viable, you'll know it well before election day.

So vote for the candidate in this election that intends to do away with FPTP.