I believe the spouse is eligible for the SS amount that is greatest between the two spouses. So if your SS is 1000 and your deceased spouse is 1500 then you would get 1500. I might be wrong but you can find out with a google search. Of course SS only kicks in when you reach eligible age. Good luck.
Up to a day before date the visa expires. No other limit. However of you don't enter within the year the the visa expires and you have to start the visa process again. I entered 4 months after visa start date without question.