I don't think they're necessarily charged for data usage (as they have dedicated servers and aren't cloud-based to our knowledge), but reducing the amount of data in memory would reduce some load on the systems. At the same time, I'm not sure if what you're suggesting would help with that (especially since most of the more expensive pieces should be living in memory as references rather than independent objects--or else what are the devs thinking?).

I'd be very curious to see how they have it coded and organized. The idea that characters move around between zones seems strange to me, when the more practical solution would be to put coordinates on the character object, and change memory references on the character and zone for one another.

Idk. I get the sense that Yoshida isn't a programmer, and that, to explain things to him, the devs had to use language that isn't technically precise (which is why some things he says about server load and such don't make a lot of sense from a programming standpoint).