If your brake system is like I think it is...are the rear adjusters a 5/8 bolt that rotates a little disk that moves the wheel cylinder screw in/out? Pictures would help. If so, the adjusters can be sourced. The disk portion is swaged onto the bolt. Careful mig welding will accomplish the task, just don't get any heat on the tensioning spring behind it. It can be done while on the vehicle but it sounds as though you nearly have the backing plates off at this point.
1) the banjo bolt and copper washers are still common items found in a true auto parts store or online if you only have the big box auto parts stores. The bolt can be reused if still serviceable.
2) the terne plate "gasket" should be used as the soft tin helps to cushion torque transfer while sealing. Get a good part number and go on the hunt.
3) if the axle shaft to hub lock plates are not present or impossible to find, you can run without them. I use good new locknuts and a little bit of the soft setting locktite...and frequently check them. I had some of the lock plates you are looking for, so they do show up once in a while.
