Am I justified in my frustration?

Looks like it will take a lot more heat. I've used one of those asphalt/weed burner type propane torches to good effect on frozen suspension parts. Lots of heat, really fast should help crack up the rusted bits. Just a thought, may not be worth the cost if you don't already have one (or have no other use for such a thing) and a propane tank..

If you go for round two, a strap around the tree will help prevent damage to it.

If you cut the end off like Yellow rose suggested, you may be able to at least salvage the third member..