Which race would be ideal for a HEALING magcro in pvp? I understand what the benefits of each race provide but which would be most beneficial? (Nord?/ Argonian?/ Breton?)
Any mag race is fine. Use high regen sets then lower slowly until you are comfortable with the sustain. Or use meta sets like olorime and get sustain from glyphs and cp
Probably Breton, though I guess Nord could work well if you can get decent enough sustain through gear and enchants. Magicka Necromancer sustain is quite bad compared to other classes, since the only "sustain skill" (Mystic Siphon) is hot garbage, and the "sustain passive" gives back less magicka than it costs to trigger it.
My magcro is a khajit because her Feline ambush passive (increase crit damage and crit healing done by 10%) synergizes well with the 2 crit chance passives from necros. You could try that.