I am guessing that when you talk about your house, you're talking about that floorplan of yours, right, and not a city-scale house symbol?

The issue with rotating a floorplan is the way CC3+ shows the outlines. When you rotate vector entities (like your floor and walls), only the outline of the entities are shown for performance reasons. But the downside is that when you have two entites on top of each other, their outlines cancel each other out. And with floorplans, you almost always have that case, because you have the floor entity and the walls entity which are in the exact same positions.

There is no other tool that can be used, but one workaround is to use the :CC2LINE: tool (or :CC2BOX: or similar) to draw a simple guide line somewhere close to the feature you want to align, and when you rotate your floorplan, make sure to include this line when you rotate. You should find that this single line will be nice and visible in the rotation preview. You can then delete the line when you are done.

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.