Thanks for your tests [:)]

Meaning that changing to SVG window paths at the same time will delete the others?

Of course not, it means that if you select two or more items in the furniture list and try to modify them, then the Customized cut out shape fields for doors and windows won't appear in the modification dialog box.

I don't think you should be too afraid of including the more advanced properties too (especially ID which is a pain to add manually), just put them far down in the window saying "optional/advanced".

I didn't include these properties in the Furniture Library Editor because I fear they would be complicated to understand and handle, but because it would take some time to program a modification dialog box able to display and manage these properties in the Editor. But if you and others are interested, that should appear one day...
At the opposite, if you need to edit IDs, I can easily deliver to you a special version of the program that will let you edit that information (it's just an hidden information).