I ended up in Darkfall Cave, not knowing it was part of the quest Touching the Sky, and now I appear to be trapped. I'm guessing that something went awry after I obtained the Darkfall Cave note, because it was shortly after this I realized I was going in circles and consulted the walkthrough on the Elder Scrolls Wiki (wikia), which says "the path leading west from the camp is an exit path, leading to a small passageway and a spiral staircase that leads to a pull-chain activated door...". Um, no, the path leading west from the camp leads to a dead end, at which I can look up and see the sky through a hole in the roof. I've managed to "mountainhop" my way up the hold, but it doesn't actually lead to the outside world like it would in real life, instead I go into polygon purgatory, or whatever you call when you're seeing bits of the world you're not supposed to. Spiral staircase? that would be lovely if it would get me out, but I haven't seen it.

Another thing, the walkthrough I mentioned says that after pulling the chain near the bed, the big room I get to contains the Chantry of Auri-El, and I looked at the image on the Chantry's article, but that looks nothing like what I saw. There's some sructure, but it's embedded in the ground, not much below the roof is showing. Does this Chantry only get exposed when the mission starts?

This is seriously the first time in my 215+ of playing this game that I've had to save the game and come back to it later, I'm completely befuddled, and a little desperate.

You can't reload a save right when entered? If so, use that save to go back outside.

What the wikia stated is correct. Give it another try.

As for part 2, it is part of a quest, thus inactive.

I've done this before but you should be able to backtrack and get back out the way you came. Helps if you have a torch or use magelight, as it can be very easy to miss doorways or paths that are right next to you.

I'm guessing you're on console so can't take screenshots? I'll try to take some for you.

I'm guessing http://i678.photobucket.com/albums/vv146/gmosko/TESV2013-09-0708-54-06-68_zpsd468533d.jpg is the structure you think is the Chantry? It's a chapel and you can't go further this way until you have the quest and meet Gelebor here.

Go back out the way you came, past the troll camp, and climb a bit until you get back to a http://i678.photobucket.com/albums/vv146/gmosko/TESV2013-09-0708-55-42-34_zps9a5afabb.jpg with a bloodstained human camp site. I'm looking back the way we came, and the path you want to get out is right behind me, so after this my PC turned around and went straight. Up that path is http://i678.photobucket.com/albums/vv146/gmosko/TESV2013-09-0708-57-15-61_zpsb1d82e57.jpg. At the skylight, turn right. Vilkas is looking into the passageway- the skylight is on his left in http://i678.photobucket.com/albums/vv146/gmosko/TESV2013-09-0708-57-19-69_zps5614216f.jpg. This is one of those you can be standing right next to and not realize is there. Also as you go further along it may look like a dead end, but there's a path that climbs. http://i678.photobucket.com/albums/vv146/gmosko/TESV2013-09-0708-57-46-40_zpsaa69acdb.jpg I'm looking back towards the skylight and the path continues on up to my right.

Eventually you'll come to a passageway http://i678.photobucket.com/albums/vv146/gmosko/TESV2013-09-0708-58-18-48_zpsd3591284.jpg. Almost there. Keep going straight. http://i678.photobucket.com/albums/vv146/gmosko/TESV2013-09-0708-58-52-69_zps22d0787f.jpg is the way out. Helps if you have a quest arrow activated that will show you the outside entrance.