First, like everyone else late to this party, I'm going to have to voice my safety concerns. And just who do you think you are, why haven't you contacted the PROFESSIONALS?
Second, the first block has an H, roughly a 2 letter gap, an L, a one letter gap, and an R. I don't know how everyone has missed this, but OBVIOUSLY you have found HITLER's grave. Congrats.
Ok, now that I got all that sarcasm out of my system... I'm mad at you Brad. First because I am SUPER jealous os your adventure, and wishing I was closer to possibly help out, and second because I just stayed up for 3 hours reading this thread from the beginning!
Sadly I don't have much to add the "tree" and letters/runes are what I would focus on with the first block. There are a TON of other marks that may or may not be man made, but those ones CLEARLY are. If I were running the show (which I'm not), I would have 3 goals. 1) do some sifting, knowing that finding some NA artifacts will brighten your day. 2) if possible, experiment with possible casting options for the etchings. Assuming the mounds are made of the same mud as what is in the rest of the cave, make your own mounds, draw a pretty picture, and see what you can get to stick to it. 3) experiment with torches and the ceiling. You mentioned it does not seem like a "solid" roof, and have noted some odd markings on the roof inside. I'm thinking that since the roof is loose, someone holding a torch near the roof causes thermal expansion, and then when they leave, the sand/dirt/pebble roof cools, contracts and falls to the ground, leaving a dimple in the roof. That would also explain why there aren't scorch/soot marks on the roof - those areas fell to the ground. I'm thinking you take a tiki torch type thing, set it up near the entrance, let it burn to 5-10 minutes and see what the roof does.
Finally, I'm going to agree with your skepticism on the saltpeter idea, not only due to crowded workings space, but also the fact that metal detecting turned up nothing. If there was an active mining operation, even a small one, I would expect to find SOME metallic evidence - broken tools, nails, screws, SOMETHING. On that note - I'm pretty new to detecting, but I'm pretty sure this is a "less is more" situation. If you are still looking to detect more in the cave, I think something like a Bounty Hunter Tracker IV might be what you want - just turn it on and dig every beep. Plus, if you would really like to put a nail in the "saltpeter" coffin, you could test the soil in the cave. I don't know how you'd do that, but my understanding is that saltpeter caves get regenerated after a few years, so if there aren't high levels of nitrites in the cave now, there probably weren't in the past either.
Last (ok, I know I said finally, but leave me alone, ideas keep coming to me), when weather or schedules don't permit, it might be worth some library time. Being optimistic, I'd start with any "Treasure Legends" and work my way down to "Local History". Plus if you are able to get in contact with any previous owners, you never know what they might be able to tell you.
