The Jade argument is rather hollow. Nothing of Jade magic he's actually going to learn is going to make the province as a whole any more fertile. He can't supercharge the waystone network, he can't simply throw more resources at the problem, and he's probably never going to be an insanely powerful wizard given how he'll have to split his focus.
The role the Jades and the food supply play and the importance therein is so obvious that failing to recognize it should be a sign of criminal incompetence- it doesn't need the Jades spending years hyping it up and using it to elevate their prominence for a ruler to realize that. The only thing the Jades can actually promise is maybe increased productivity from state subsidies for their work and decent PR for a wizard. They made a bunch of promises but there's not really substance to them. And then we have the shitshow of Jade traditionalism. None of that can possibly be good for Mandred as a child.
As for the Golds… I distinctly remember Hubert wasn't the only problematic elemental wind wizard in our merry band. Johann definitely had his share of problems with his College because of the exact same things Gehenna sees in Mandred. Even if it can be moderated and kept under control, we'd just be hoping for special treatment Mandred gets to paper over the inherent problems.
All we need to do is turn a boy into a functional adult who's both well disposed towards the colleges and a highly visible example of what kind of people actually make up the colleges for outsiders. We just need to focus on what works best for him- not 'for a capable ruler', not 'for the good of the nation'- most of that should follow if he's even mildly competent as a symbol and advocate.