I removed the set nut from the TA-10 Symmons valve assembly, but the spigot valve won't unscrew more than half a turn. I don't want to snap anything or damage my pipe, but I put a lot of pressure on unscrewing it and it just won't budge more than half a turn.

See this video.
https://www.youtube.com/watch?v=HBC_Qhw10aQ

See these instructions.
Scowl down to page 6 "Service"
Note instructions in paragraph 4.
http://www.symmons.com/~/media/Files/Symmons/Products/Temptrol/TemptrolTubShowerSysZV89ins.pdf