Why do Betsy and Jeff stop by to see Susan? Why was Susan alone in her house?
Betsy needs her to go along with her alliby and answer the phone.

Susan is alone because she didn't want to go to church with her family that night.

What is surprising about David's grandmother's death? How does Susan figure this out?
Susan didn't know Mark would kill her and David does not own a brown sweater however Mark does.
Why doesn't Susan go to the police with her suspicions about Mark?
Betsy and Jeff stop her and run out to tell Mark. Mark said he would handle it and that's when Mark did all that stuff.
Do you think Betsy and Jeff believe Mark killed the old woman? Explain?
No their words say they don't but their actions say they do.