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.

1. When you hit the add-on buttons the program loads some defaults to ensure that things work sensibly. For example, if the overland filters were kept when you went to the DD3 add-on, all catalog buttons would just return empty results because the overland filters doesn't make sense in a dungeon setting. Unfortunately, this also means that the settings loaded with the map is lost. Generally, it is not adviceable to use these buttons in the middle of a map. If you wish to load symbols from a different add-on, use :ICON_CATALOG: instead, this allows you to load any catalog, not just the one provided by the defaults for whatever add-on you switch to. The easiest way to reload the correct settings is to just reload the current map, you don't need to restart the program. Simply save the map (if you have changes you wish to keep), then simply pick the map from the top of the recent file list in the File menu. This will reload the map, and reload the settings from it.
2. One way of making torn edges is to place a white polygon with some fractalized edges on top of everything. The white will cover up part of the map, appearing as a tear. Add some inside glow and perhaps some blur or similar to make it look more like a tear and not just a flat poly.

Your step 3 is the problem here. You shouldn't be selecting the same line again, it is already part of the combined path. By doing that, you are basically telling the line to combine with itself.

Once you have done 1 & 2, you are ready to use Path to Poly.