Hey Billy, hey I cant fault you for doin things on a budget, I'm kinda like that myself and try n make do with what parts I have available lol THAT BEING SAID
1) When I was a design engineer, and best I recall NO WARANTY, when I specified individual insulated conductors for use in wet, damp or underground applications I BELIEVE IT WAS THWN and NOT THHN Conductors that were preferred/required. The practicing Electricians will know if thats correct, my memory cells are stale lol
2) The Neutral GrounDED Conductor carries the same current as the Hot phase UnGrounDED Conductor, so you want it to be as big as the hot phase conductor.
3) Its permissible to use buried conduit with internal wiring, Im NOT going to open the can of worms about depth and where it needs concrete encasement and how you need rigid risers when it comes up out of the ground lol
AGAIN yes it can "work" and with only a resistive heater and a light bulb voltage drop isnt so critical. Theres the NEC and UL approved and safer wiring (which I learned and had to practice) and then theres Billy Bobs wiring that can still "work"
If using paralell conductors to increase ampacity and reduce voltage drop, they should be the exact same length and wire type and of course, in the same raceway.
Id still use a GFCI feed and use 3 wires (or paralell runs), Hot Phase UnGrounDED Conductor,,,,Neutral GrounDED Conductor,,,,,,Equipment GroundING Conductor IF YOURE USING 120 VOLT ONLY
In a deer stand you realy need a 400 amp 480Y 277 volt three phase four wire system lol lol
PS one of those cheap t stat controlled 240 volt baseboard strip heaters (or any 240 volt heater) would be a good way to go to reduce current. If you need 120 also then youre back to 4 wires however. Orrrrrrrr maybe 240 volt heater and 240 volt lights (even a couple 120's in series) so you only need 3 wires, 2 Hots, Equipment Ground AND IT DONT PULL THE AMPS AS IF RAN AT 120 so you save on wire and voltage drop !!!!!!!!!!!!!!!!
Hang in there, if you use a GFCI feed and reasonable wire size you probably wont wake up dead even if its NOT per the NEC,,,,,,But I still dont like to go on record and advise otherwise grrrrrrrr