Golden Wyvern Adept
Not explicitly spelled out on magicians not being regents, but the text under wizard implies it very heavily. And wizard has rules for bodyguards of the regent, while magician doesn't.I don't think there's any rule preventing a blooded magician from being a regent, but it's true that he shouldn't be able to do anything with sources (except prevent others from using them).
Being stuck between Avanil and Boeruine would be a challenging situation for PCs, but Tuornen is in the same situation and would be a more interesting domain than Taeghas.
Weird little detail in "Other domains" : while Brosengae is "recommended", the Brosen Royal Guild is "not available for PCs"... even though it belongs to the regent of Brosengae.
I missed the bit on the Brosen Royal Guild, which is usually the sort of thing I like to comment on. Brosengae alone doesn't give a full picture of the the duchess' holdings. Really, places like that should say, "not recommended, see the full listing at the end of this chapter. Endier's a good example- Endier on its own isn't much, but you have Guilder Kalien's whole Heartlands Outfitters, and that's a realm.