California can't leave on it's own, but it will definitely have leverage to oppose some of a Trump's bad policies. Both need each other, but California basically subsides the U.S. If CA is doing bad, the country is doing bad.
Yes. California is highly self-sufficient. It has enough people to keep its economy going even if the US decided to shun it. I'm surprised to know that it has higher GDP per capita than Sweden even with more than 4x the population. The US economy will likely be surpassed by China and the EU if it loses California.
However, NO ELITE would be in it for this. I can't imagine either party supporting it. Showbizz would be SO AGAINST it. This would get zero support.... so I guess it can happen then? Considering the ongoing trend on the world of going against expected results.