The two lhbs near me, (25 mi. radius), both carry the nut extract wine tannin: http://homebrew.voelckerbeverage.com/wine-tannin-p/ld6300a.htm
"...from European/Spanish Chestnut tree"
I'm not really worried about long term storage, I don't see this tasty beverage lasting too long...
Hmmmm... Never noticed it contained nuts! I would suggest just ordering the stuff from EC Kraus or another online shop nearest you. For me, the online shops nearby often ship day of order...
I am sure your patience is getting to you though, I totally understand...
I could be wrong but I don't think you absolutely have to add the tannin when you pitch yeast. I think people do it in secondary oftentimes, especially after tasting. Before aging I believe is the key.
Pom rind? Maybe. I tend to go with things thought that I know what it tastes like. Crushed Pomegranate or pomegranate juice would probably work as well. I would try the latter because it's easier, cleaner, and there's probably juice from a dozen pomegranates in one bottle.
I would vote for the steeped black tea at this point. Like I've said, people have had success with that in meads, apfelwein, etc.
Regarding the rubber band, I would say you're good to go to airlock. That is petty vigorous, but a blowoff tube you would for sure be fine, airlock I think you would be OK too. If you can monitor it for a few hours that be best. If the airlock gets clogged, go to a blowoff tube.
And do know, if your house heats up on a hot day or you turn on the furnace, that will increase the vigor of the fermentation and it could blow off at that point. But if I were a betting man I would say you're probably good.