Magic works differently depending on the user and the type of magic. Fel is the most corrupting force in the universe, thus why people can become demons fairly easily. The same isn't said for arcane, where it gradually changed the Night Elves to Nightborne in Suramar.
Xavius was made into a satyr by Sargeras himself. Sargeras took Xavius' soul in the Twisting Nether and shaped it, making him reborn. He then gave Xavius the ability to change others - which is once again fel energy.
Naga came from the magic of an Old God - one of the most powerful lieutenants of the Void Lords. Not nearly the same as arcane or fel at all. There's a difference between exposure and a curse.
Once again, fel and arcane are different schools of magic.
The Warcraft film is a separate universe from the WoW game. Thrall's birth was different in the movie, and many other characters have had different origins from the movie to the game. In-game, Thrall was born green, because fel energy gradually corrupted nearly all the orcs on Draenor by simply being near it. In our universe, Gul'dan never corrupted Thrall in order to make sure that he would remain alive - Gul'dan wasn't ever near him in our universe, and Thrall wasn't stillborn in the "main" universe (which is the game). So, no, that's not canon, at all.
The Arcan'dor has all the ley lines in Suramar redirected to it, making it very powerful. The Fal'dorei were created by a giant burst of arcane energy at once on Nightborne. Notice how the stable Arcan'dor simply turned the Nightfallen back into Nightborne, and didn't change them into another entirely different race altogether.