There's no direct way to find them, I do it by a bit of structured trial and error. I start by figuring out which sheet contains the problematic entities by hiding half of them, figuring out if the problem still persist, if not, it is on one of the hidden sheets, if it continues, it is on the still visible ones. Continuing that way until I am down to one sheet (Takes more effort to write that explanation than actual do it in practice)

Since there where quite a lot of entities on that sheet, I narrowed it further by hiding layers. Could have used the same approach, but I found it faster to do a list of all the entities on the sheet to figure out which layers were involved, and then tested those 1 by one.

This lead me to the SYMBOLS sheet and FURNITURE layer if my mind serves me correctly.

Still a bunch of visible entities, so I just proceeded erasing a good bunch of them at a time, retrying between each batch. Turns out that the problem still persisted after deleting all visible entities. And that makes it easy to find the culprits, since the last step is now just doing a info -> list, then hit a (or right click o get the selection menu and select all) and do it. And voila, the list command shows me the culprits.

I have no idea why these corrupted entities show up in the first place. I've never had that happen to me personally, but maps with corrupted entities like that do show up in the forums every now and then. And this also brings up the point why it is so difficult to fix those bugs, they're nearly impossible to reproduce. A corrupted map posted by a user don't show how it happened, and the developers struggle to reproduce it so they can find and fix the issues. I don't find anything familiar with the way you describe CC3+ yourself. Granted, I have experienced crashes every now and then, but it's not something that happens often. And my maps have never corrupted.

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Best way to handle that is to make sure the layer the grid is on is frozen. That's usually the HEX/SQUARE GRID layer.

• Yes. Anything you do in-map (using allowed resources) is perfectly fine. The main thing here is that you cannot find your own symbols/fills from outside the allowed resources, because this causes distribution problems for the atlas.
• Yes, that is fine.
• Yes. I usually prefer to have these on different maps, because CC3+ can really only maintain one scale in any given map file, but there are cases when things work best together.
• Nothing against tokens directly (as long as they adhere to the allowed resources), but generally having monster tokens on the map makes the image export a bit weird, anyone downloading just the png file from the atlas would have a image file with immovable tokens.

Am I understanding you right here that each sheet is one floor, so that all the floors are in the same map file?

In CC3+, because of things like sheets effects and such, maps tends to use a lot of sheets, so it is generally better to use one map per floor. Although it is possible to keep it as one map too, but then the macros hiding the sheets must be updated to hide the complete set of sheets. With some nice naming you can do this rather easily by taking advantage of the HIDESHTF and SHOWSHTF commands which hides and show sheets by a wildcard filter, so for example all sheets for floor 1 could be named FLOOR1_SOMETHING and so on.