the kid twist is sssoo 'out there' because when he got on the plane he was sonless and if they never got to the island... well.. that isn't 15 years...
I think you are misunderstanding the alernative timeline. It isn't that things changed during the plane ride in 2004.
The timeline split off at the time of the bomb, in 1977. So loads of things that occurred since 1977 have lead them to be different people entirely in the new version of 2004. And you need to think of it like a butterfly effect. Since 1977, no one is on the island, so all those people have affected someone in the real world, who affected someone else, etc. So in the new 2004, Locke's dad isn't a complete jerk, because something is different in his past since 1977, which means Locke hasn't had the complete breakdown he did originally, which means he has stayed with Helen, and on and on.
The writers have made some things the same -- Locke is still paralysed but probably in a different manner this time -- to show that some things are just destiny but otherwise the characters are different.
