Hey all. I was able to work out a great repair for this figure's easily broken arm socket. I'll walk through it here in case it helps anyone.
So here's the break. It started with the this flimsy piece of plastic which seems to be the only thing that actually holds the entire diecast arm to the torso/body. It's literally just two small plastic tubes that a screw runs through, connecting it to a hinge in the body. Mine snapped off without me even noticing at first (until the entire arm fell out of the body). Then, upon opening up the figure, I found there was damage to the left arm's hinge as well, so that likely would've broken off very soon as well.
First I had to open up the torso. There are just two sets of screws holding it together. But they're both hidden under seriously glued down panels. The first set is on the front of the figure under the trapezius armor pieces. I carefully pried them off using an exacto blade working carefully across them, between the plate and the silver body they sit on.
The next set is in the back, just under the flaps. I carefully slid my exacto back and forth under the silver lower portion of the flap until it popped off. Then slowly used the exacto to pry off the screw cover underneath.
I didn't worry about the the revealed underside and screw area. They look roughed up when revealed. But it mostly disappears once the plates are glued back on at the end (seen below) and even more so once the red armor cover is placed over all of it.
Here's the right arm hinge, after I glued the pieces back together. The prongs pointed at by the green arrows are the only thing holding the arms to the figure. Poor design for sure.
Here you can see the left arm hinge breaking apart already as well.
After gluing the pieces with loctite super glue (sparingly, it can make plastic brittle if too much is used), I grabbed some spare aluminum I had in the shop and began to cut and shape it to wrap around those flimsy plastic screw ports.
I went ahead and reinforced the left joint as well. I used aluminum strips around the hinge joints and across the top for extra stability, adhering them with loctite as well. I made sure to keep the aluminum smooth and flush with the surface of the plastic so they fit smoothly into the ports on larger socket piece.
Next I install the arms back into their joints with the new reinforced pieces.
Reassembled, it's as good as new. Perhaps even better, since I'm pretty sure the metal reinforcements will make those plastic joints even stronger than originally designed.