Engine problems are so tough to diagnose on a forum. I'll assume fuel is ok and there's no air leaks.
Ok, first thing. You said it will run when the igniter is on? So if you remove it will die? If so that's a bad plug. The filament is weak and can't retain heat on it's own. You shouldn't need to leave the igniter on the plug. The plug will only get so hot but not any hotter due to the igniter being attached. In your case all it was doing was continually heating a weak filament. Get a new plug, preferably a hot one.
I personally haven't had any issues starting cold motors in 6 degree or below temps. But if you don't have the ability to pre-heat it just before starting I would recommend keeping the head wrapped with something to retain as much room warmth before attempting to fire it up outside. It certainly won't hurt.
It may also seem like your needle settings are off. Incidentally, yes, both low and high speed needles will have an effect on idle. Close both needles fully and re-open both 2.5 turns. Try again and see how that goes. This should give you a good baseline to tune from if it fires up. If it doesn't fire up there may be other issues at hand.
Good luck sir and report back.
ETC: Serpent 411 Eryx 3.0 |Serpent 411 Eryx 2.0 | Serpent 411 TE | Xray T2 | Tamiya F104 V2
NTC: Serpent Cobra GT | Serpent 811 | Serpent 748 TQ | Mugen MTX-4
Heli's: Align TRex 500 EX | HK 500 x2