"...chain, which is now a couple of links shorter than ideal"

If your chain is too short to fit onto your big-big combo, you are riding your bike in a perilous condition. If you forget (and you will) and shift into that combination you risk severe damage to your derailleur and possibly your frame.

The first thing I'd recommend is to get a new chain and make sure it is long enough. Then worry about the derailleur.