1. Yes, if the flight you want to SDC to is oversold at T-3, there's a good chance DL will change it to Y0.

2. I assume CAK-ATL and ATL-LGA are separate tickets? I don't know if the agent processing the VDB would be able to "combine" them like you're asking, but you have nothing to lose by asking. VDB is a negotiation--you can ask for whatever you want, and the GA will tell you if they are able and willing to make it happen or not.