Am I justified in my frustration?

I'm just trying to help you from this point on.
Getting pissed off and continuing to struggle with how in your mind you got screwed over isn't going to resolve what you have to deal with now that it's over.
And it is.
Quit complaining.

There's nothing wrong with getting someone to build a gear set/third member for you.

If you used my method of cutting a window with a cut off wheel in the back of the housing, you could cut the axle shaft close to the third member and pull it out.
Then take the caps off and throw away the gear/bearing assembly as well as the pinion and bearing.
That leaves you with the bare third member ready for cleaning/blasting and rebuild.
(A core)

Then put the housing in a vertical press and push the remainder of the axle out.
And if it doesn't come out that way even with lots of heat on the housing flange area above where the bearing runs, then cut off the axle flange above the bearing area.

If you cut the flange off, then you might as well have it narrowed to A-Body width and make a few bucks on the bare housing.
Weld the saved window back in and dress the welds at the same time.

There's more than 2 ways to skin a cat.......
If you understand this concept then that would be the way to go. Metal is just metal. If you can cut it apart you can put it back together. If you're not used to doing this kinda thing it can be a bit scary and intimidating. The only thing I'd do different is I'd use a torch to cut the window. But either one way will work as long as you can handle a torch well.