1) So it sounds like she sold her main home for $200,000 and sold the rental for $400,000.

2) Fair Market Value on the husband's date of death in 2021 was the same as the selling price in December 2022?

4) You will need to see the 2000 and 2001 tax returns to see if depreciation was taken on the building in those two years.  If depreciation was not taken those two years, you need Form 3115 to "catch up" on the depreciation, and that Form takes some time to learn how to do it, and more time to actually prepare.

There is a 50% step-up in Basis on the date of death, so you will need to factor that in for the depreciation in 2021 and 2022, as well as for the sale of the rental.

This isn't a particularly easy return if you don't have experience with those things, so you may consider passing this one to somebody that has done these things.