My 2 cents is that the two Westlife songs don't really sound too much like Thank You For Loving Me to me. For instance, when you listen to It's My Life and Larger Than Life (Backstreet Boys) back to back you can easily see how it has fingerprints from the same person. I don't get that feeling from these examples.

And much like it's been suggested with Save The World, it does sound like they were trying to make a 'Bon Jovi by numbers' type ballad, adapted to 2000, and these were the two attempts. The lyrics sound like Bon Jovi too.

That being said, Jon is definitely not above buying songs from other people, so it wouldn't totally shock me. It's a nice theory, but I'd take it more seriously if there wasn't a bunch of other songs that credit outside writers, as others pointed out, and if there was a single reputable source that would list Mac and Hector as writers, which doesn't seem to be the case.