Is the idea of beings affiliated with multiple realms covered anywhere? For example, could a faerie be corrupted by an Infernal influence be both Faerie and Infernal for a time, or would it just immediately flip from one to the other at some point?
Considering a diabolist magus, it seems such a magus would need to draw on both Magical and Infernal realms, rather than just one or the other. Same goes for those with strong links to faerie.
In the case of magical creatures/spirits, rather than magi, how does belonging to multiple realms affect their Might score? And are they more vulnerable in that magical attacks affecting either realm will affect them, or less vulnerable in that magical attacks affecting both realms will be required to eliminate them completely?
Humans can be affiliated with more than one realm.
Merinta magi with Faerie Magic are, for example, affiliated with both the Magic and Faerie realms.
It is impossible for a being to belong to more than one realm at a time. So any being with a Might score will belong to one, and exactly one, Realm.
A mortal creature (i.e. no Might Score) can find itself aligned to multiple realms, as you've pointed out the case of Diabolist Magi. Other examples include Merinita (Faerie & Magic) and Holy Magi (Dominion & Magic).
In the case of supernatural creatures, there are some cases where we can see the influences of two realms intersecting. Homunculus (RoP:F p. 118) is a faerie speculated to reside on the border between Faerie & Magic; it gives you a Faerie-aligned Gift but also gives you access to Faerie Magic.
Chthonic creatures are Magical or Faerie creatures with domains/reputations that can easily get them conflated with Demons. One suggestion on handling such creatures is for SH&H to detect them as unholy, even in the absence of actual Infernal corruption (see Ancient Magic, Hyperborean Magic p. 127-128). See also the segment on Chthonic Magic (RoP:I p.123).
There are even cases for Magical or Faerie creatures worshipping God; they will either keep their original alignment or if they are devout enough, they could potentially have their Might alignment shift to Dominion.
Hell can also easily corrupt creatures of other realms, and the transition from one realm to another takes time, as was presented in the case of Maju (see Faith & Flame).
Despite these influences, the Might of said creatures is always aligned toward a single realm. Homonculus is Faerie; Chthonic creatures are either Magic or Faerie; a creature corrupted by Hell will keep its original might alignment until the corruption has been completed, at which point it shifts its Might to Infernal.
You cannot affect them with two different kinds of Might-removal or Wards for two different realms; only one will actually work. It's always an either-or-situation.
I seem to recall a story about a being of Magic switching to another Realm in a previous edition, but can't recall it. I think there are suggestions that Magic Forests can switch Realm to Faerie if they are sufficiently reduced by such an aura.
One idea I like is for the faerie God Apollo to have switched to Magic, which is what caused the Cessation of Oracles.
Hey, in 5th edition we have one of these having a chapter in Antagonists.
Antagonists has a Magic Dragon that changes to Divine.
Through the Aegis has a familiar of the magic realm that was once an Infernal snake and isn't sure how he became 'redeemed'.
Faith and Flame has a Faerie god which is becoming closer and closer to incarnating as a demon.
Can't think of any others off the top of my head. (Ouroboros, thanks for reminding me of the one in Antagonists; I had completely forgotten I was going to use it to plot-bother our saga's wandering monk.)