I'm very interested to see what answers you'll get. I started the same kind of "not world record but for fun" segmented run a few months ago, ans making a topic like this one might have saved me from starting over a couple times.
From my experience, everything hud-related (like speedmeter or yaw/pitch) can be removed when playing back the demo, but in-game stuff (like bxt_show_triggers 1) will stay, so you have to make sur it's off.
As for the opening doors stuff, it only works if you go in the map by map transition, and not by loading a save already in it. This means that every npc trick will require you to do the entire playthrough from the last map transition to the door opening in a single segment. Sometimes, like we've got hostiles, this is easy. But the infinite health door segment will be hard because it's super long.
As for save loads, those are pretty random I think. Most actions you do during them will be delayed until the end of the load. But some actions will sometimes straight off crash the game or send you straight to menu. Having a too low fps_max (10-15) will usually throw you to the manu on any and all loading screens, at least it does that in my case.
I'm looking forward to the answers to your other questions