On Nerevar's face being the Indoril helm:

The Indoril masks were official, and they each depicted his true visage. There was also a special Daedric helmet version in the Morrowind Art Book, but its look depicted his more terrible aspect as Hortator and Padomaic champion.

I may say lots of things, but Lord Indoril Nerevar the Hortator was my beloved from the get go during my tenure as MW's Art Director.

Edit: to Lorus, that's his bonewalker version, lost to the annals of most Tribune histories. Nerevar, while betrayed or not, was still dear to ALMSIVI after death.