Well... after taking everything apart (Again!!!!), inspecting, futzing and experimenting, I managed to get it working again, with all the upgrades installed (excluding the enhanced hammer spring and rocket valve, which I traded for the new hammer assembly).
I'm not exactly sure what did the trick... It could have been the reassembly of either the hammer assembly or the slide internals, or perhaps the removal of the paint on the tracks inside the metal slide (to allow for smoother action in conjunction with the hammer assembly and lower frame's rails), or maybe even the few drops of oil dropped in a few select spots. Who knows...
I wish I knew exactly what fixed it, but either way:
Hooray for
full-metal full-auto wonderfulness!
I'm not religious, but God, if you're reading this: Thank you.