How does it work for a First Lady after an assassination? How long does she have in the White House afterwards? Is there a policy in place that allows her a sum of money to help after her husband's death?
"A presidential widow is provided a $20,000 annual lifetime pension and franking privileges. The widow must waive the right to any annuity or pension under any other legislation. "
This does not specify that the president was killed in office, but I believe that the rules stille applies. This rule comes from the Former Presidents Act of 1958, which was implemented for the sake of ex-presidents dignity, after Truman almost went broke. Trumans widow is the only presidential widow to use this pension.
https://www.senate.gov/reference/resources/pdf/98-249.pdf Not the legislation, but a summary and 'briefing' of sorts for congressmen.
As for how long a former first lady would have to move out. I think I can almost guarantee you that there is no 'rule' per se. This would be settled between the F-FLOTUS and the new presidential family.