I think you're over-reacting. The manager was caught enforcing a vague no handbags policy, and gave up when you called her on it. While she is the manager she is probably not the source of the policy - she certainly wasn't the source of proper or equiptable enforcement. If you return to this theater you will probably not be called on the presence of your Jumbo again. I suggest continue going to the movies as you usually do, and if you get called out again for carrying your Jumbo, then take it up with whatever level of management you want.

fwiw the Jumbo is advertised as a great CCW pack, maybe the manager recognized it as such. If they had a no firearms policy, it would have been correct for her to point this out to you, and ask for any firearms to be kept outside the theater. (Since this is a purely hypothetical issue let's avoid the discussion of whether a public theater can or should enforce a ban on firearms).

JohnE, the ADA has nothing to do with this situation, he wasn't asking for an accomodation (seating for mobility impaired, assistance for hearing impaired etc) to view the movie.