I am not sure how images would work. The lines for the original lines are just plain simple lines. The easiest way to deal with them is to open up the symbol catalog for editing (@Symbols\Cosmographer\basesym.FSC), the from symbol manager, clone the symbols, and finally, go in and edit the two clones, using change properties on the lines to change their width. You can then save the symbol catalog with both your new modified copies and the original in them, and you should be able to easily pick from the starmap options which of the two to use.

(I normally don't recommend overwriting the original symbol catalog, but for this case, it may be the simplest, providing you do clone the symbols and don't change the originals. But if you prefer to be safe, either save your modifications under a new name, or make a backup of the original first.)

Node editing as Jim explains is a good option. Another good option is to use combine path to merge the walls back into one entity again. Combine path is explained in this article (you'll want to skip steps 7 and 8, as those are for turning it into a polygon, which you don't need)

Unfortunately no, this command only allows up to 10.

The alternative would be the Symbols in Area command. It doesn't pack symbols tightly the same way, but by experimenting with the random arrangement you may be able to get close. While that command only lets you choose a single symbol, it has the advantage that you can ask it to use all the symbols in the same symbol group as the one you picked, technically allowing for using as many symbols as you want. There is a blog article dealing with setting up a custom symbol group just for this purpose.

Sure. No need to break anything down for the atlas. I prefer dungeon levels being a single file each.

Not sure how you manage to grab things that are on hidden sheets. I've never been able to do that myself. But do note that if you hide a sheet, then use a drawing tool or other command that needs to operate on that sheet, it will become unhidden, and then entities on it can be selected.

In any case, when it comes to hiding/hiding the grid, I recommend doing it using layers instead. Make sure the grid layer (usually HEX/SQUARE GRID) is both hidden and frozen. (and never unfreeze/thaw it when showing it, it should always stay frozen, as this prevents it from being accidentally manipulated when you are working on something else.)

To show/hide the layer from the command line, use the TOGLF command. This will hide the layer if visible, and show it if not. This command accepts a wildcard filter, so you can simply type

TOGLF GRID

to toggle the visibility of the HEX/SQUARE GRID layer. (Of course, the filter means EVERY layer with the word GRID in the name will be toggled, but you usually only have one).