Yeah, 5 oz is too much a lot of the time. I generally only end up using 3.5 oz or so.
Decarbing bottled beer isn't something that I would try. I'm sure it would be doable, you'd probably just want to figure out what temperature to get the bottles down to, allow them to stabilize at that temperature and then pop them and recap to clear the head. Just be careful to scale the amount of sugar correctly for your now smaller bottle arrangement.
As far as carbing the TAD system, what I've done in the past is to make two separate batches of priming sugar, one for the TAD bottle, the other for the bottling bucket. Add the boiled sugar to the respective vessels (make sure you cool the sugar for the TAD) and then just siphon in your beer.