It's perfectly legal, unless there are some additional details not mentioned. For instance, if it amounted to discrimination on race, or was in retaliation for unionizing. What would be illegal about it? California can't just force a company to stay in one place. Companies move offices, even headquarters, all the time.

Your math would be covered by what's known as a relocation package. Often, it's a basic lump sum to (theoretically) cover the costs of moving. You can either accept it or not. Same for any pay adjustments that may come with it.

Layoff isn't a legal term. The closest would be terminated without cause, which is exactly what this is. Since California (along with every state that isn't Montana) is an "At will" state, this again is perfectly legal.

It's a shitty decision, but there's nothing stopping them from making it.