I hate how derogative that name is. I wish we could have changed him back in the course of the game itself, but alas, was not to be.
Uh, but you can change it to Glenn later on using the Nu in 65,000,000BC's Laruba Ruins. At least...I think I've got that village's name right...I sometimes get those confused...
Personally, their names mostly fit...Robo can't be R66-Y because the party says it doesn't fit (or at least Marle does)...So, I guess you could as a slam against Marle...
Marle can't be Nadia because she's hiding out and not telling her name yet...afterwards, it would be alright to change her name to Nadia, I think...
Frog too doesn't make sense to immediately call him Glenn because he says "Frog will do" because Lucca was calling him that. Afterwards it makes sense to change his name to Glenn though.
Magus though, I think, is the exception to that rule. I don't think it really makes sense to keep on calling him Magus unless you believe that's his own personal choice. I think after hearing his story on the Cape, he should be called Janus.
As for the curse...I think that it still goes on until Magus is seperated from Frog temporally, ie the gates are shut so Magus has no connection to Frog, thusly, the curse is lifted whether or not Magus is slain. That's my take on it anyways...