Minecraft – How to prevent the chest from being sat on

As Ben Blank said, the only thing that will spontaneously cause an explosion, if you haven't made any TNT, is a creeper that got into your house. I see three possible directions for the creeper to enter from:

1. The caves

   Most likely, either you have an insufficiently lit patch of floor in the connected caves (walls and ceilings don't matter), or there is a dark branch that you haven't found. I strongly recommend always installing doors at connections between caves and living space.

2. The front door

   The damage you described seems to be focused around the doorward side of you. You say you removed "the bricks leading up to the door", but if there is still a one-block-jump path it is possible a creeper got in anyway. Next time, don't worry about removing the path, just close the door! If you have a wooden door, zombies can break them - this will cause a loud sound of someone punching some wood.

3. The ceiling

   You haven't said anything about your ceiling design. Is it possible that your ceiling had a hole in it through which a creeper entered? There are [minimal spoilers] ways for dirt/grass blocks to go away, so if you had a dirt roof perhaps a creeper fell in.

Miscellany

To my surprise, it appeared that nearly all my objects were still there. Most were sitting in the same spot that their chest was in. As best I can tell, I was able to recover nearly everything, with the exception of the signs and 2 of the 5 chest blocks.

Chests drop their contents when destroyed, as you saw. If there had been two explosions, then the second one would have destroyed all of your resources, as they have almost no “health”.

They also tend to leave big craters, and there seemed to be too little structural damage in this case.

The size of an explosion crater depends on the strength of the materials. If you meet a creeper outside, then it will leave a large hole in the dirt, but the same explosion will do much less damage to stone.

Oddly, the door and torches outside were gone, but the structure itself was untouched save for two holes in the floor (marked "Gone" in the above picture).

Wooden objects are generally weaker against explosions, and torches have no blast resistance at all.

Yes! And very easily, too. Profiles allow you to tell Minecraft where to store all its files by changing the Game Directory option in the profile. Just add something memorable onto the Game Directory (such as 1.7-snapshots) and that profile will never see your existing worlds.

How this works is, setting this to a different folder than the default .minecraft folder makes the game keep an almost entirely separate Minecraft install in the specified folder. (The only thing it doesn't keep separate is the game code itself – this is still drawn from the .minecraft/versions/ folder, which as you've found works just fine and doesn't compromise firewalling different installs.)

I use this myself to maintain multiple private modpacks separately from the main Minecraft version, and they co-exist perfectly and peacefully within the official launcher. It means that there is absolutely no danger of accidentally opening a modded world while running vanilla Minecraft (and thereby destroying the save!) or vice versa. I make it easy on myself by simply adding modpacks/[name-of-modpack] onto the end of the default Game Directory, which neatly stores all my modded Minecraft stuff in a "modpacks" folder within my .minecraft folder.