PS3 and PC, accounts, links and stuff

[Niwashi]

So if I change the PSN to one I make, I can login in the game with my account.

Provided that you buy a PS3 copy of FFXIV to register to your SE account, then yes. You cannot use the same FFXIV/PS3 registration that's on her account when you log into yours. You have to buy it twice in order to use it with two different accounts.

If she wants to play she has to login with her PSN and then her SE account and so on.

Yes.

Linking an account that started in PC to PS3 won't make me unavailable to return PC, isn't it?

No, provided that you've bought both the PC and PS3 versions, you can switch back and forth between them whenever you want. (You just can't be logged into both at the same time.)