It's usefull when you want a set of shades from a different base color than the ones from the default palette.

You can change the default wall to be whatever you want. The :DD2WALL: button just starts the drawing tool named Wall, Default. You can edit this to change the properties (advisable to make a copy of it first to preserve the original, but if you want to edit the tool that is used by the button, you of course have to edit the original, and just keep a copy under a different name as a backup). To make it cutable, all that is required is that it is on the WALLS layer. From your images, you are using SS4, and I don't seem to have a problem with doors cutting that wall though.

To make them into single walls, you need to do some manual work. Erase the entire wall (one of them) where there is a complete overlap, and use tools like :CC2BREAK: to split and cut parts out of walls where you have a partial overlap and don't want to delete the whole length of the wall.

In addition to what Sue is writing above, check for double walls. When I look at the screenshots you post, the door is clearly cutting something, otherwise there wouldn't be a white rectangle there. And the wall fill pattern in the second image is offset from the first one, indicating that there seem to be two walls on top of each other there. (The wall cutting functionality only cuts one entity)

I think you may be looking in the wrong place for the files. FilterTest2.dll is found in the Filters directory inside the data directory (C:\ProgramData\Profantasy\CC3Plus\Filters), but this is not where the actual dll's reside (unsure why that test dll is there at all). The actual dll files are inside the Filters subdir in the CC3+ installation directory (C:\Program Files (x86)\ProFantasy\CC3Plus\Filters) (and there is no FilterTest2.dll there).

Try creating a new map and see if there is any difference.

Modern Floorplans is from SS3: Modern.