As far as your "inverse corner ride" question, I have never been able to survive on anything besides a 90 degree corner or something close to it. If it's slightly more obtuse (wider) like say 100 degrees you can survive but just barely and you usually die pretty fast.
The checkpoint stuff can be frustrating, yes. However, I should explain how to get a checkpoint anywhere you need on Metropolis for the corner riding stuff:
Once you get the banshee through to the last map, fight until you destroy the first wraith. You'll go through a tunnel and reach a fountain/waterfall thing. After passing through this area you'll get a CP and the battle in the street loads. At this point, you can fly anywhere and you have several minutes to get setup for a CP that ALWAYS comes regardless of what happens with the AI in the battle.
If lack of checkpoints are ruining your SLASO attempts, I can only offer my thoughts which you might already know. Jumping and the melee attack delay or completely stop a checkpoint from happening. I guess try to avoid doing either unnecessarily.