Any idea as to why my Mac won't allow me to remove Furniture.jar?

Probably, because the program is still running or Sweet Home 3D.app folder is protected. There can't be other reasons.

has there been (or will there be) a release of an SH3F file containing default furniture?

Hans, I think that by "default" JaimeLF means the furniture that is available in Sweet Home 3D default catalog, not the additional known SH3F files.
JaimeLF, I can write an Ant script that would build a SH3F file for the default furniture. Why do you need it?