It's reasonably standard. The last money in gets to be the first money out. It solves the problem of a startup taking money from a new investor and then immediately disbursing it to the old investors (or other similar shenanigans).

The existing shareholders have to vote to approve the new round. They wouldn't do so unless they thought it overall worthwhile.

Nope. From what I've heard it is standard practice for later rounds to dilute the investors from previous rounds.

It could point to that. The founders wanting to take some home with them after a 6 billion dollar offer is likely. I'd pocket a cool $50mm. It would keep my head in the game, but it would definitely let me sleep better at night in case it turns out to be a trend, rather than a sustainable business.