voting validates a system I am opposed to

This sentiment resonates with me. Although I do vote, I steadfastly believe that the United States is an oligarchy.

Coupled with plenty of young voters that just don't care, I find it unlikely they will put Bernie in the Whitehouse.

I'd point to Occupy Wall Street as proof that they do care. Regardless of the effectiveness of the drum circle as a means of economics and political reform, a lot of man-hours went into that. Apathetic people generally don't invade the financial district.

He doesn't need to convince them that the political system works. He needs to convince them that the window hasn't yet closed for them to mobilize and fix it. He needs to convince them that the window is closing, not closed.

I think that both Sanders and Lessig's decisions to run were attempts to do that.

I also think it is possible. Ron Paul's bottleneck was being libertarian, but he showed that strategy has merit. He brought in disenfranchised voters and captured 10% of the Republican party. These people didn't just vote for him, they tried to learn the system and take over the party's local infrastructure. If America had been in a different place culturally, they might have had the numbers to do it.