As has been said not all in Mhach and Amdapor were in favour of the war and massive disasters have a tendency to give people a new perspective. Coupled with the witch hunts it's not surprising that White Mages and Black Mages banded together and chose to craft a new form of magic, drawing upon their combined knowledge, that perhaps was not so dangerous (it'd be a bit stupid to continue the exact practices that caused the whole mess in the first place).
Also doubt there's a whole bunch of Red Mages out there as opposed to a few scholars keeping the tradition alive. It could even be like sch and smn where you are researching into a lost art. Nothing about the description confirmed they are active today just during the 6th Umbral era at least.
As for Sharlayan they've not been shown to have access to white and black magic (and I doubt they'd want to touch the latter with a 10 foot pole) so them creating a discipline based on the two seems a bit of a stretch.