Bummer of a problem! Removing a 124 expansion valve isn't too bad if you don't run into something like this.

If you drill out the stripped bolt, you will be able to remove the expansion valve. It will be a bit more trouble than usual; you'll have to move the liquid/vapor lines further out of the way than is normally required. Otherwide, no big deal.

I'm pretty sure the back plate can be removed once the expansion valve is out. It's roughly an 'H' shape, where the four legs seat behind the flares near the end of the evaporator inlet/outlet tubes. It may be a bit fiddly to get it out of there, but I think it can be done. Should be able to remove the broken bolt from the H plate with just pliers once you have it on the bench.

Good luck,

- JimY