There are two versions of Gauntlet out there -- the "unlicensed" Tengen black cart version, and the "Licensed" grey cart version. IIRC my Famtasia WIPs were done with the "Licensed" version. When I did a bit of testing on FCEU, I'd switched to the "Unlicensed" version for legal reasons (I own the black cart).
anywho, level 10 and 20 both have a shootable wall next to the door. although i said i wasn't sure about 20, actually i think shooting the wall at 10 would be faster than picking up+using the key, whereas in 20, the key is on the route anyway, so it would take less time.
(PS: Ignore the position of the character; what's important is the position of the hole in the wall. ;p)
24+78: i still think you can shave those frames off.. IIRC you can hold down start while it's fading in, and as soon as it finishes fading in, you can release and it'll start to fade out, without increasing the HP counter. it might not even save a frame, but it certainly looks more aesthetically pleasing to keep everything uniform.
there might be a few other things in the middle (the invisible levels, etc), but i'd have to dig up my notes about what items are in which levels, to see what's "necessary" and what isn't.. given that i'm at work, and it's been 8+ months since i last looked at the game, i'm not in prime position to pick out improvements.