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.

That would be my preferred solution for the poles, yes.

I really loved the map showing all 7000 of the rivers feeding the Mississippi. Kind of cool how it collects water from a really huge part of the US.