So, testing, discovered a couple things:
- Rallying 10th drone to opponent's base gets it there to hatch at perfect time
- Protoss CAN get a pylon in position to power both a wall and hatch-killing cannons though, at both positions:
I think the 2nd pic (upper right/lower left spawns) is a more likely forge expand position, since the 1st pic (upper left/lower right spawns) requires 3 buildings to wall off and I'm not a Protoss wall expert or anything but that seems cumbersome and unlikely. And furthermore, the pylon at a normal wall position can power the cannons, whereas in the 2nd pic, the Protoss would have to be thinking about this ahead of time and purposely place their pylon farther back than they normally would (I imagine).
As such, my new plan is...
- lower left/top right spawns: scout w/ 10th drone and see if forge expand and if pylon is close to wall or far from wall - hatch their natural if conditions are right
- top left/lower right spawns: play normally