If I used Wall of Stone to surround a creature, then used my additional panels to close off the top so the creature is totally encased, then ended my concentration on the spell, what would happen? Would the creature be crushed under all the falling stone or would it just turn to dust or something?

If you drop concentration the spell ends and the wall just disappears, so no effect.

If you want to collapse part of the wall onto a creature you need to maintain concentration while you destroy the supporting panel(s).  This will be slow as each panel is AC15 and has 30 HP per inch of thickness.  Since each panel is either 6 inches thick for a 10x10 foot square, or 3 inches thick for a 10x20 foot rectangle, that will be a total of 180 or 90 HP respectively.

You're probably better off attacking enemies directly and using Wall of Stone for crowd control.

Dang it. I was hoping to use it as a trash compactor with my enemies being the trash.

Hmmm, if your DM allows you might try some of the Dunamancy spells from Explorer's Guide to Wildemount which are about manipulating gravity.  Gravity Sinkhole perhaps.

Wait. What if I had an ally use Move Earth on the top stone or supporting stone?

Quote from Snowtworf >>

Wait. What if I had an ally use Move Earth on the top stone or supporting stone?

 Move Earth on constructed stone??? Won't work mate, sorry.  

DANG IT!

I might be mathing this wrong, but that 3-inch supporting panel (assuming granite. It only says stone, doesn't specify WHAT stone. Maybe I could vary which stone and have the supporting panel be sandstone and the drop cube be granite?) would need to support 171,706.2787 lbs. Would that be enough to just... break through a 3-inch slab?

Edit: In theory I can also make it 2 of the cubes stacked right on top of each other, doubling the weight needed to support. Also... It theory it's TECHNICALLY stable... for all of 0.5 seconds... BUT TECHNICALLY correct and that's the best kind of correct!

Rules and Game Mechanics has nothing to do with "mathing" real life conditions  into the game environment. It deals with what the rules actually say as to how to moderate a situation.

The idea you have might be a better fit for General Discussion, Tips and Tactics, DMs Only or Homebrew. If you are looking for opinions on whether or not others think this idea has merit, you should state that. This falls squarely in the realm of DM moderation as to how things can go but, that's not to say that you can't use actual rules and examples to promote your point of view.

And don’t forget, with wall of stone, if you try to surround them they get a saving throw to try and move out of the way 

If a creature would be surrounded on all sides by the wall (or the wall and another solid surface), that creature can make a Dexterity saving throw. On a success, it can use its reaction to move up to its speed so that it is no longer enclosed by the wall.

I'm assuming, for this question, they failed it. I mean, if they succeeded there'd be no point in asking.

What I think you want to do is create a horizontal wall that mostly fills the room you are in with only a single panel connected to it. Like a popsicle. Then find a way to put enough damage into that single panel to make the rest fall down.

You might be able to find a way to crush them with a second spell after walling them in with the first spell, but I don't think you have found it yet.

If they are in heavy armor, maybe your DM would let you drown them.  But remember the second spell can't be concentration.