Added a few EDITs to the original post.
Thank you very much, THT! I did indeed read almost all of them. But there's a few things I didn't remember that I can use, like reserving some T/D/R to some Houses. I'm not sure about the increased magnitude, since I already nerfed magic a lot by giving less points to buy arts and spells at character creation, no good books, and other limitations. I guess that may be enough.
Yes, that's one bonus of choosing such an early date
But regarding the excessive power of hermetic magi, I was thinking about Parma Magica. I can't believe nobody smuggled it outside of the Order. Especially in the first decades, when there was a lot of hedge wizards joining and too few hermetic magi to control everything. A vitkir, say, could join the Order and then teach the Parma to his old colleagues shortly after. No civilization has been able to keep a technology secret for long. Sooner or later everything leaks.
Thus I will probably allow some non hermetic magi to have Parma, later in my campaign, when my PCs will be stronger. Members of the Order of Odin, or the druids, or the witches and shapeshifters of Pomerania. I see that Parma Magica was the first invention of Bonissagus, and I understand you don't need to know a lot of Magic Theory to learn it.
Of course the seek and destroy program will be one of the main concern of the OoH, as well as the identification of the moles.
This is not very clear in the books. Somewhere it looks like it was the Tribunal in 773, somewhere it's stated that it happened in 865. Probably the second GT defined the regions, but it wasn't until 865 that the regional tribunals were allowed to have their own laws (which couldn't contradict the Order's laws, of course).
In 790 most of the covenants are either in the Rhine tribunal or the Diedne area (Brittany and british islands). Other regions (the Alps, Rome, Transilvania, Provence) have probably just one or two covenants each, and thus there's no need for regional tribunals. So, the Grand Tribunal will be the only one, for some time.
Yes, indeed it is. The Order as described in Sub Rosa is already very similar to the one we use to know. I guess that's also because the magazine tries not to diverge from canon, and the official manuals don't go into the "what could the Order be really like, during its childhood?"
In fact the infos that I found most useful in SR16 are those regarding the pagan people and their religion.