It does sound like a piece of clear tape is the 
only thing you can really do for the rip. But if 
you want to be extra-nice to the cel, you could 
use acid-free clear tape. It does exist, although 
it's expensive.

And far as the framing goes - in the future, get 
the "professional" to cut you the frame, the 
glass, and the (hopefully) acid-free mat board 
and backing board and then YOU put the cel into 
it yourself. You'll find its also less expensive 
when you don't have them complete the job. That'd 
also give you more reason to use up the special 
acid-free tape you bought to pick your tear. ;)