My Steam playtime is 55 hours, and my save time was just over 50. Between deaths and reloads to not waste resources and ruin searches where I didn't save, I guess that difference makes sense.
Some of the puzzles were definitely difficult even with all the hints (that endgame mystery I posted about [sp]only had one tablet that barely helped[/sp], the clues of [sp]the Cog puzzle[/sp] were tricky to put together, etc.). Sometimes I suffered for not making note of certain text. And some things I mostly came across by accident or trial and error. I think it's probably a similar difficulty to the La-Mulana remake.
I don't know anything about a Hell Temple equivalent at the moment, but I would guess there isn't one (yet) since they probably just wanted to get the game released.
About garb:
I didn't find that one. The one I found was pretty early on (in fact, I'm sure you came across it too, as it's very close to a mantra mural you definitely have), and I returned there much later, and it was open. So I guess some item must've opened them? It sounds like Kickstarter backers could get a file to put in the game directory that opens them immediately, though.