That is some good stuff. It can be hard to recover a save. Feels so good to go through all that and find out the save can be used again. That is a really good accomplishment you got there.So, I have somehow fixed my issue by deleting the NBT data it had stored for my character, and doing the one small MCEdit change.
I am now happily playing on my old world, with none of the old issues I had, so I think my NBT was fucked up for some reason. More playtesting will reveal if more issues arise. Anyway, yeah. I deleted the random number/letter nbt in my player.dat folder, but still somehow have my inventory unaffected. Seems legit.
MC Edit is a pain to learn enough of to find and delete a block. I tend to forget what I learned the previous time. (just remembered, check your light levels. I have had cases of the light levels being forgotten while working with MC edit and Minecraft refusing to update them until their is a nearby block update.)
NBT data can be corrupted if multiple accounts log into the same single player save on the same instance/PC. Minecraft saves a lot of character stuff in the level.dat, along with other places. My brother managed to acquire a morph on one direwolf 20 save for every account he accidentally logged in into that save on. Along with that his inventory would randomly disappear. Playing on a server prevents a lot of the issues that can crop up in single player. It is a pain constantly having to setup and update servers on the same PC the client is on. I haven't noticed the computer specs mattering a whole lot. Though I wouldn't recommend a computer that has trouble playing vanilla minecraft for trying it on. Anything better then that should be able to work fine with it.