Q1:Keyboard and Trackpad stopped working

A:Both the keyboard and trackpad signals travel to the logic board over the one cable assembly- Trackpad Cable so it’s worth the expense to replace it first. But before we do that how does the battery look? as a puffed up battery can cause problems with the cable. Was someone inside before this happened? One other possibility is a damaged logic board connector so when you take the cable off really inspect the contact pins didn’t get damaged on the board side and I would also check the cable end side as well.

Q2: “A” and “D” on keyboard not working after replaced trackpad

Sounds like a bad keyboard the keys A & D share the same row line with S but they use different column lines! so Q & E should also be affected too, if not then the given keys are bad. You may need a new keyboard.