After much agonizing, I decided to use soffit plug vents, instead of the vent strips. This means sticking with the angled facia (instead of plumb), but much less expense.

After removing shingles, planking, and insulation from over bathroom, the attic floor is CAREFULLY extended toward the front of the house, not resting at any point on the old bathroom walls or ceiling.

And the rain and tarps never end...