Question about endgame exploring

So, I've finished the game with the alternate ending. Actually by accident since I fell down the sinkhole and didn't die.

I've gone back down on cave 7 to get the modern bow, but since going back requires the climbing axe or quite a bit of glitch jumping, I figured I'd just skirt through the sinkhole and lab again.

Problem is: the lab entrance (where you use the first keycard) seems to be blocked by a cave in. How do I get back at all? Is it possible to go all the way back to C7's entrance without the climbing axe or do I just kill myself somehow and hope I get taken to cave 2 like on the usual deaths?

Edit: actually made it out with a couple of close calls while jumping across climbing gaps. Plus got the tennis racket, so all is good.