10%. They will all be offered about ~3 months salary (depends on tenure). Some will stay and try to work through it, some will succeed, some won't and only get ~1 month salary severance now. Amazon is really aiming for 6% actually fired, by first trying to fire 10%.

Because the rational choice is to take the high severance (being handed the PIP means your manager and manager's manager have already made up their minds against you, I've seen a coworker given an impossible task), that means that being an average Amazon engineer will have a 50% chance of being fired by your 5th year.

I have no idea. I don't work at Amazon. Typically in the tech industry, departures tend to have severance included, unless it's for something egregious (eg violence), so I would imagine it does include severance. But again, I don't work for Amazon, so no idea. :)