All three buttons (the one for Checkpoint 7 and both for Checkpoint 8 ) will remain pressed after you restart, but will reset if you die anywhere from Checkpoint 6 onward. Dying prior to C6 is "safe" because the zone with the buttons is unloaded.

I assume this is also why the lazers at the end sometimes don't spawn. They're still turned off form the prior run so long as you don't die from C6 onward.

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.