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i'll assume the attunement slot is only "occupied" as long as the spell is in effect, and it would make sense that the target of the effect (if it positive) would be the one with the attunement slot taken (though if its something like polymorphing a frog and making it permanent, it would be the...
The intent, it seems, is for 8th level or high casting to remove a previous restriction. For the attunement thing, that's a bit more tricky but I'll assume it would be the target of the spell.
I will say i do like EvilDMGuy's idea to connect permanancy to the attunement mechanic. Makes it both reasonable and balanced with you having to pick against having permanent truesigth against a magic item.