Guardians of the Temple of Mythal

This is something that's never explicitly explained. If, as Solas says, they were only placed on the faces of lowly slaves by their opulent owners (especially as defined by Solas when he provides his explanation of ancient vallaslin), there's really no reason why Abelas and the other Sentinels should have them as well. If this was Obsidian or Bethesda, I'd presume this is a clue that we'll get to hear the other side of history and the other historical account that makes this ancient conflict more three-dimensional than how Inquisition and Trespasser defined it (since the Evunaris are nothing more than one-dimensional villains given how Solas described them).

Yes, I think it is all a matter of perspective where Abelas and the sentinels are concerned. They are bound to the will of Mythal, just as the Inquisitor is bound if they drink from the Well. This does not mean it was forced on them any more than it was forced upon the Inquisitor (except by the plot). They chose of their own free will to serve Mythal. However, to Solas that is no better than slavery as he makes clear when he remonstrates with the Inquisitor if they do drink from the Well.

What we have yet to discover is if nobles marked their own faces as a sign of allegiance to a particular god or whether they had bare faces because they were free to follow any or all of the gods and their loyalty could shift according to circumstances. Nor do we know if they marked the faces of their servants to indicate they were their property or simply to show they belonged to their household as loyal retainers. However, Solas and his followers considered them the marks of people who were subservient to the Evanuris and therefore effectively slaves.

I'm sure he meant Mythal; it likely happened due to spell-check auto-correct.

It probably thought you were trying to type the Mughal Empire.

I think they wore those marks long before the Tevinter wars, way before humans discovered magic.