Goat magic is banned, not so much because of abuse, but because it marks you as a badguy and I wanna saga of heroes and good guys. Also, Vortigern was trying to impose his own personal Gnostic theology on the game. Arawn chaffed against the setting too.
To spell it out clear for all: the Divine is Holy and Good and reigns supreme. That is just the rules of the setting. I made the mistake of allowing Command to be Tainted but not Infernal. But Ludo did other stuff that clearly marked him as evil. Vort got bent outta shape because I wouldn't allow Ludo to use Goat magic on angels.
If you indulge in evil you are damned. If you transform to a faerie or a daimon, you loose your soul. This has no bearing on actual game play tough. If you die you are dead anyway and it doesn't matter what you or I or anyone thinks happens afterward.
The potential abuse Fixer is worried about isn't the might (Ludo had fair scores to justify it, but chaffed when told he was overspecialized and could not open the Arts of an apprentice with so many zero art scores). The potential abuse is that you can cast it over and over again and have a legion of might 10 faries at your service.
I do not thing these spells can be naturally permenant. Your best bet is "Until (Condition)", and set the condition as something a sufficient long way off.
I am not so worried about potential abuse. But I made Fixer the Rules Shark for a reason. The Ludo situation demonstrates I am too lenient and allow players to do things just to placate them without considering long term ramifications.
My thoughts on Constant effects for Items: Still subject to MR. However, they are not "cancelled" by an Aegis or such. Leave the area and the effect revives and does not count as a "use" for the day. Not being subject to MR is a rabbit hole I do not wish to get trapped in and is a severe break from RAW that may have ramifications I have not considered.
BTW, I am glad your philosophy is to keep the Dominion in check rather than opposing it. I can work with that.