Right now there are quite a few Republicans who back the Lincoln Project. But if Warren (or Sanders) was the VP pick, then fiscal conservatives of that group would either abstain or go ahead and vote Trump.
No they won't. They will still go with Biden if he picks Warren. They have already endorsed Biden. Those who are part of the Lincoln Project can't stand Trump. Their goal is to defeat Trump in 2020. They are with Biden regardless of who he chooses for his VP.