Kylie was killed because she went to help Gemma.
I think Clayton stole money from the kebab shop and came back for more.
Gemma was telling him to go away and he kicked off. Kylie heard the shouting and went to help, and he stabbed her and ran away.
Clayton is now in prison. He'd already fallen out with Shona, so she doesn't get to visit him.
For a long time only Gail knew she was Clayton's mum. When David found out he didn't want to see her but they got back together. The children know but say she's not to blame for what Clayton did.
I have no idea how David and Kylie bought the house, but somehow they did.
The house is clearly TARDIS-like, but this is entirely normal in soap opera land. The granny-annex/burial ground has helped with fitting more people in. It's unclear who shares which bedrooms upstairs.
I think David helped with the burial, so yes, he knows all about it. They managed to blame Jason Grimshaw's now deceased father, Tony, for the murder and body burying, as he was building the granny annex at the time of the murder and conveniently died shortly afterwards, so was unable to clear his own name.
It's possible Todd knows this, I can't remember, but if he does he took the view that Tony was dead and Sarah isn't, so he chose to keep her secret.