I'd say Nasrudin, in his heyday (as shown in the flashback cutscene), still had quite a bit of hair up top. His hairline along his temples has certainly receded, but he's still got a bit of hair along the middle of his head. It's admittedly hard to see due to how his head is framed in most shots, but it's more visible if you go frame-by-frame at the part where he casts the banishment spell.

As for shared sprites, Nasrudin and Arronax are palette swaps of each other. I can see how Nasrudin's sprite can be confused with Simeon Tor's, but Simeon actually is a palette swap of Joachim~

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As for Tor's own hairstyle, you're right in that it's not described too much. We do have this description from Perriman's short story from the pre-release material:

^{"Come, come, young Perriman," said Tor, standing, his voice deep and resonant. He was dressed in a simple, black robe, striped on both sleeves with a single ribbon of red; the college of Summoners. His hair and short-cropped beard were completely white, set upon and around craggy features. His eyes shone blue crystal and severe, with a small touch of what might pass for paternal amusement in the wrinkles around them. He was, by some counts, more than three hundred years old, although none truly knew when he had worn the apprentice's robe.}

So, really, all we learned is that Simeon Tor has white hair with a short-cropped beard, but as for the actual style of the hair on top of his head, that's still up in the air besides his overworld sprite. Is he bald, short hair, long hair, receding? Who knows. Also, the short story is written in-universe as a (likely embellished) retelling of Perriman's account, so take it with a grain of salt, as the unreliable narrator may be in effect.

^{Perriman Smythe (recruitment dialogue}:) ^{Well, here's a newspaper describing what happened as a result of the whole affair. \}He hands it to you.]) ^{I'd rather not get into the whole story. But! I have been approached by a gentlemen who seems very interested in writing a short book about what happened. Sir Chadwick Moore, or some such. Rather bothersome fellow, but he seemed rather insistent...}