Where did you get the 30 years value? As far as I remember reading, guardian role its for life, dismissal or death.
I think there's a reference about Guardian System was establish after Maya kidnap event, so: 2062 minus 2096, well, you are right its 30, 34?? to be accurate.
About their duty and the life long term: The Head of House have authority to regulate, approve, move, dismiss or whatsoever think its best to the House, Sakurai died not cuz protecting Maya (her duty), she did protecting Tats, other guardian and outcast from Main Branch, I not saying its like that with Tats, but far I read on some parts, like Vol1 reference about being tool provider and not guardian and Maya order on Vol 8, made me think, they have been trying to "assign" another role to him, which btw is not Guardian.
About Miyuki marriage, I don't remember exactly where but I think there's a part where She ready think about it and the implications in her relations with Tats when that comes, add to this I think Miyuki is posesive type, so its not like she will not give Tats to other Woman, like say: Lina or Honoka, but her sense of love are very in that are among others, with Honoka at least, she kind of tricky, I not sure with Lina how goes cuz still lacking info, but She "approves" Honoka knowing shes not Tats type of woman could raise his interest, on the other side she on defensive and over aware from Mayumi, in the beginning Miyuki suspect from Mari and Mayumi, but later she focus on Mayumi, as I said before: Miyuki actions acknowledge Mayumi as "love rival" or at least someone whom could awake or ready did, Tats interests about woman's and that's too much to say.
I think the author ready knows about psychological cases where patients don have emotions, like rage, love and similar s or being too logical, if you have he time for classic, please see "Raiman", with Dustin Hoffman and Tom Cruise, with this you people will known, Tats conditions can be "restart" to get the concept of "emotions" and probably feeling, if we take like say when on the conference center during Yokohama incident he stayed and no leaving to not left behind Leo, Ercka and co, is clues about his internal changes.
EDIT: About High Psions counts context on Vol3 its about Who could do magic now and prequirements to do, so before in time a really few people whom could cast magic and now is little bit more easy to do and that means more people could do, so in the context We been talking its the performance of Magicians could have high psions counts and the ones don't, still, people with that traits (like Tats dad) but not talented in magic, well, don't do anything, put it simple: I been talking High Psions count as will be stamina instead.