I was acting on the premise that Johnson may resign as PM, but not as leader of the Conservative Party. In this scenario he would become leader of the opposition until a GE.
Theoretically yes, if they were able to form a minority government. It would need co-operation from all of the opposition parties, so would seem very unlikely. However, if Johnson resigns as PM and an election timetable may mean falling out of the EU at the end of October, there is a small...