fn doesn't have a scancode, so AHK does nothing
is there any reason you want a TK over something fullsize?
a) I think that you are likely correct: I would expect the Fn to be intercepted by the keyboard firmware. I'm kind of a pro at AutoHotKey; it's on my resume.
b) Yes: I don't use the arrow keys, and my varidesk keyboard section is very narrow. Even without that constraint, my long-term goal is to have a keyboard (other than a laptop) where the tenkey directly adjoins the main section, both to keep the trackball closer and for overall symmetry.
On a Unicomp 104 I'm using, I'm experimenting with putting the trackball on books so that it's just above the F9-F12 cluster. Unfortunately I used books are slippery covers, but I think the idea is generally sound.
c) I went ahead and ordered one. Office Depot has them for around $80, which is far more reasonable than Amazon's $160.
PS: I need to update my signature.