Necromancy and Lores of Chaos don't give arcane marks. They give mutations. The Colleges are quite firm that there is a difference. Collegiates can also get mutations. The Hag Witches and Ice Witches get witch marks. Priests get divine marks.
Dedicating yourself to a single wind wasn't invented for the Empire by Teclis. In Cathay humans are only allowed to learn one lore: Heavens. Their alchemists seem to be split between Fire and Metal. The Indic books that Mathilde went over evidently didn't say something groundbreaking, like humans using multiple winds without going insane. Hell, if the Sidestory is to be taken seriously, the Old Ones themselves noted that humans had a tendency to get attached. There is only one tradition in the world that is known to produce humans capable of safely using multiple Lores and that is Albion.
I don't think there is evidence that arcane marks come from prolonged use of a single wind. If anything the Collegiate system is evidence against that. The Colleges like arcane marks because they ensure that the apprentice can't take what they were taught and leave. You don't need to be deeply immersed in a wind to get them and the first arcane mark that people get often won't be achieved in deep immersion. You only need to fumble casting once. I can't think of a reason why she would be so shocked about a human having arcane marks.