i've never seen the gunner not spawn, and i've reset right after hitting the button. is there something more to it?

Skeletons spawning feels very random to me. Haven't really looked into it since only grey knight cares if they spawn or not and I'm not manly enough to run that yet.

Shooty knight's aim is always the same, but he can have faster movement speed (which allows for better checkpoint 6 start). As far as I know he gains the extra speed in his very last transition in checkpoint 8 when he moves to his last resting place (murder by spikes).

Once the last button is pressed, the lasers guarding the last checkpoint will never come back up -- even if you die anywhere after C6. That's why I couldn't figure out what exactly affected the other buttons.

If what you say works, though, does wonders for future runs! Inconsistency with said buttons and minigun knight is what put me off grinding for sub-8:00.