I suppose that depends on the entity which grants the spell. Can you imagine a spell which stops working in the 43rd floor working outside the tower? We know contracts are void there and they might be related to or be spells. If all contracts are void in the 43rd floor then that probably applies even to the immortality contracts the family heads have (which goes to show how few fucks gives about tower residents). I suppose there are entities great enough that their power would work inside and outside the tower but my bet would be against guardians in this case.
Maybe? It's hard to say what would happen to that. And it would basically benefit people whose affinity is towards powerful bodies, like zahard. Gustang would be kinda fucked since without shinsoo he looses basically everything in this scenario.
Well, look at Karaka's armor by example. It's a spell related item and it seemed to me like it worked quite fine there. So i think that it depends on the spell, or, contracts are no spells at all.
But would the armor work outside of the tower? I have got no idea.
Would Hoaqin's soul devouring power work outside of it?
If Guardians can use spells and Arlene was a spellcaster before she entered the tower (was that part ever mentioned?), some of their powers might keep working outside of it.
Since we know of no one who left (maybe Phanta or Enryu, but that matter isn't really clear Edit: And Arlene of course, but although she might be dead -which would mean that she was free of the immortality contract once she got outside- we do not know for sure) the tower, you are right, it's just a guess. But i can't think of a reason why it shouldn't work. Sure, they would probably age again, maybe even faster than the average person (depends on the contract issue).
That's why i picked Quant as my example. If his body is like steel, i am pretty damn sure that Gustang's is at the very least at that level (most likely way higher).
He got spells at his disposal, looks like a genius and was able to hone his skills (which are not all shinsoo related) for thousands of years. I think he would do just fine (except if he runs into monsters like Urek or Axis or...?).