When you pay the $35 fee and get a working replacement, you get 14 days to be satisfied with that replacement. I've had a working replacement, replaced within the 14 days (A replacement, replacement Evo 3D - screen stuck up out of the bezel, improperly seated), and did not
have to pay $35 again. Your mileage may vary.
It always helps if you are a pleasant customer and treat the associates how you'd like to be treated, not just because you want a fee waived or get something in return but because it is the right thing to do. (not directed at you, just a general friendly reminder)
Have you tried a factory reset from the Bootloader menu yet (Fast boot off, then power on while holding volume down button, navigate down to factory reset)?
It worked for me and got my 'data call failure' handset to go through activation again and fully back on the network.
This is a multi-faceted issue though, I'm suspecting there is not one fix 'to rule them all'.
This whole thing troubles me. Random, "unexplained", errors that boot people from the network and Sprint seemingly not knowing/being able to do anything about it in multiple cases other than swap a seemingly perfectly good handset for another one...
Mod note: Is there any chance that this issue can be streamlined into one topic and possibly re-titled???