man thats too bad ,i feel for you ,im the same cant leave loose threads alone with out ruining things ,
im not a expert and i dont have any of the dark knight figures ,but have messed around with many other mezcos and there knock offs ,
it might be possible to if not repair it ,at least make it more acceptable to your eyes ,
firstly does the glove section rotate ,
if so its a male ,female plug in joint ,all the ones ive disassembled have this set up ,if it doesn't rotate ,its glued in place ,it can still be separated by the method below but will take a bit more care and effort
you should be able to separate them ,i usually put the figure on a warm radiator to soften it up, this gently warms it through and hasnt in my case caused any damage to the fabric suits ,that i worry about with the hairdryer technique, and then grip just about the sections i wish to separate and pull gently but firmly ,you have to be careful as you can separate the forearm from the elbow ,which could be the cause the floppy elbow you have ,
once separated you should be able to expose the elbow and if possible push it back to firmness if that's the cause or while warm squeeze the joint to tightness and when it cools hopefully it remains tight, or tighten it with the floor polish method ive read about ,then re glue fabric in position and when its dry re warm the female side and push on to peg
i hope this is helpful and you get him back to a condition you can settle for
don't hesitate to get back to me if you need any other help