Layoffs don't necessarily lead to share prices dropping, in the short term or long term. It definitely wouldn't impact the loans he presumably lives off of by using said shares as collateral.
No they couldn’t because the severance is paid out of the money the budget set aside for wages for the year.
So out of the 12 months, they give 4 months to the laid off worker and Facebook pockets the other 8 months.
Facebook could give zero severance, "pocket the full year budget" to use your phrasing, and distribute it to owners as a dividend. The severance comes directly from cash that could have been distributed to owners, as GP originally said.
He is not putting the shares down himself. He is just subject to price fluctuations like everyone else — so how is he taking personal responsibility for it?