It's not to late to cut the hinges off and install new ones.

You say the servo is chattering?? If you disconect the pushrod, does the flap move without binding? Does it move feely in its full range of movement?

Have you checked the servos for damage? IE chipped gear.

From the sounds, you may have to much movement on the servo. The fact that you can adjust the end point where it stops chattering but chatters on the other end of the travel would say that your end points are not adjusted right. Thes may be a dumb question, but I'll ask anyway. Did you adjust both end points or only one?

By the way, epoxing the hinges won't do much for you. If you want to disable the flaps, pull the servo and put in a lug that the push rod will tie to and mechanically adjust the flap to lock in the up position.

Don