Thanks for sharing. I appreciate warning stories like this, every little bit helps make me more aware and safe on my commute.

Was the turn improperly signed, that you weren't aware it was left and straight? That seems weird to me that you wouldn't know that, presumably this intersection is on your regular commute and you use it frequently?

But also the driver's behavior seems weird to me. If I were in my car at a light in a left/straight lane, and the bike that got there before me was positioned at the right side of my lane, instead of the right side of the right lane, I would assume he's turning left, and at least wait until we're through the intersection before looking for an opportunity to safely pass.

The only way it makes sense to me that the driver would try to pass immediately, is if the right lane was right-turn-only, then it is plausible to think that you would take that lane position if you wanted to go straight.

I agree with all of your conclusions. In straight/left, take the full lane, and look to make extra clear signals with drivers. Starting from a stoplight is when cars are slowest, so it makes the least difference if a car is held back a little waiting for a bike (which might be quicker off the line anyways).