The Jades have one of the better reputations, being literal plant wizards in a province known for being the breadbasket of the Empire. Life certainly isn't the worst choice for him, especially since he won't ever get the title of Emperor. Might as well be known as a literal breadwinner.
Reikland is the centre of Sigmarite worship, which means maximum wizard bigotry, and it already being a magically fertile place means there'd be less demand for Jades specifically. And besides all that, it's peasants who'd most appreciate Jades, and they don't matter.
I don't see why the Brights and the Jades wouldn't be options for Mandred, the Greys' Vow of Poverty is deliberately filled with loopholes, as long as he doesn't abuse his power he should be fine, both the Amethyst and Light Orders could make an exception to the inheritance and celibacy rules for him, obedience to he who is rightfully elected Emperor of Sigmar's Holy Empire trumps obedience to the laws and ideals of their Order in the Articles, and while Ghur might not seem like the ideal mindset for the ruler of a civilized society we've seen Dragomas manage to adequately fulfill his role as administrator of the Colleges for 24+ years now despite his natural Ghur inclinations.
He's a man and not from a historical Druid family, so the hardcore Druids won't want anything to do with him.
I think the traditionalists would be more likely to argue that he shouldn't be allowed to join the Jades at all than to break part of the bedrock of their beliefs and have a Druid teach him.
It was the same kind of roll that made Eike a wizard. I'm pretty sure it made Mandred a full on wizard. And he full on recharged an echantment. A minor one, but I doubt a perpetual level wizard would manage that subconciously with no training.
Panoramia as the compromise candidate: She's got a connection to the tranditionalists, but isn't set against them like the secularists.
I do think they would want mandred in Altdorf, or at least the empire... It's one thing to have him roam around, it's another to have him completely outside his future lands...
Reikland, specifically Altdorf, is the most "magically accepting" place. The colleges were built in their backyard and Altdorfers take pride in being "okay" with Wizards, and those who aren't know full well the consequences of being openly bigoted against them. It doesn't matter that the current Cult of Sigmar is centralised in Altdorf, when the Emperor changes, they're moving out of Altdorf. The only reason they're here is because they want to be next to the Emperor. Their previous capital was Nuln after all, and it's where their biggest Cathedral is.
Altdorf is an exception to the rest of Reikland, and whether Reiklanders know about the magic fountain doesn't matter. The Reikland is naturally fertile, so Jades aren't that needed.
Honestly, a massive library full of lore and skill books being temporarily closed due to the librarians losing their magical ability to speak and needing you brave adventurers to retrieve a magical item granting them speech again so you can reopen the library and get the hook to the next adventure or the lore on the big bad of whatever campaign you're on sounds like exactly something I could see popping up in a game.
That sounds like a solid argument to me! Understanding the spoken component of any commands we're giving the Waystones is a pretty compelling reason.
Even down to the part where it lets you assume that they're just magically muted somehow and only at the end offhandedly admits that oh right the librarians are a hivemind of giant spiders, no big deal, here's fifty gold and a library pass for your troubles, which you will have to accept from said spiders.
