I believe vfb is correct. Pretender (and prophet) HP are determined by the province that they are attacking from. So if your pretender teleports from a dom +10 prov into a dom - 10 prov he will fight with max HP.
However, magic resistance (and perhaps some of the other stats) are immediately scaled according to the dominion of the province that the fight takes place in. So teleporting into a dom -10 prov is risky if you suspect the presence of mages.
And no, it does not apply to immortal resurrection. If an immortal dies attacking from a province with + dom into a prov with - dom, he will not be resurrected.