That super jump is a great find! I can’t wait to try that myself. I think in the original Half-Life doing that just made you slip off like it was a slope or get stuck in mid-air.
2: As for the “unfinished texture”, well, I think it should stay black. It is supposed to be dark way up there and if the flashlight was able to illuminate the ceiling then it would undermine the feeling that it stretches farther than it really can. You aren’t meant to be up there. You can see through the top of that rocket exhaust thing because it has been optimized so the computer doesn’t have to render things the player can’t see.
1: Black Mesa has a sort of problem with teaching players that they can be silent. Since this mechanic was completely absent in Half-Life 2, having them react to gunshots (and thus cuing in the player that grenades probably attract attention too) is pretty important to informing the player that those things can’t exactly see.