If I have one cell with one container and put a zillion items in that container (just throwing a large number out there)...the CS only shows the cell as having one object in it, so is having a zillion items in one container going to have a huge negative impact? Or does the cell not even take all that in account, and only registers the one container (until you open the container and then the game explodes)

Does the cell process all that information or does it ignore it?

The game won't explode unless the player takes the zillion items from the container and drops them all onto the ground. Until then they don't really exist.

Are you absolutely sure a bout that? I once placed a chest with a very large number of items in it into a cell and got a CTD every time I entered that cell until I deleted the chest.

Define very.
If a significant number of items in the container had scripts attached to them, that could be a problem.
If a significant number were leveled lists rather than actual items the game could get bogged down resolving them all.
The point in response to the OP is: items in containers aren't references, they are just entries in a linked list with a type, quantity, and possibly some extra data (in an unopened container, generally just a script). So there is none of the performance impact of having actual references to each of those individual items in the gameworld.