No problem in hopping past-carbonation. I sometimes leave keg hops in until the keg kicks. Not ideal, but still not a big deal.
No need to sanitize the bag if its fairly clean.
I would do two things:
1. Adjust carbonation gently. Slowly off-gas to add the hops, keep at serving pressure after adding the hops. Lower-gravity beers tend to get that carbonic bite if you rush in force-carbonating, especially more than once.
2. Suspend (per jeffy) and shake to get the most out of 'em. If the beer is cold, extraction will be slower. I use a bag, some fishing line, and a (new) plastic bobber to suspend keg hops.