Arabic defective verbs
They are called "final-weak" verbs in John Mace "Arabic Verbs" and "verbs with weak final radical" in Haywood + Nahmad "A New Arabic Grammar" (with "defective verbs" as a subtitle). Properly speaking, "assimilated" verbs should be "initial-weak" verbs, and "hollow" verbs should be "middle-weak" verbs, but both of the above references use the terms "assimilated" and "hollow". Both references also use "doubled" in place of what we term "geminate". If you want to rename "defective" to "final-weak" (and possibly also "geminate" to "doubled"), go ahead. Keep in mind there isn't just Category:Arabic defective verbs but also Category:Arabic defective form-I verbs, Category:Arabic defective form-II verbs, etc., and subcategories which place themselves underneath these various categories. If you rename these categories then Module:ar-verb needs to be appropriately changed and so does Template:ar-weakrootcat and possibly other places.
I don't know anything about Arabic, so I can't really say. w:Defective verb does have a section on Arabic though. Does that describe what you mean?
No, that section describes actual defective verbs in the sense of missing parts of their conjugation, not in the sense of "having a weak final radical" (where "weak" here specifically means w or y). There's only a couple such verbs in Arabic missing parts of their conjugation. So yes the fact that "defective verb" can mean two different things is confusing and I suggest you go ahead and rename "defective" -> "final-weak" in all of the places I just mentioned. You can probably do this easier than I can.