My guess is if you have the expansion on one platform and not the other it may just flat out deny you the ability to log in. There would be too many checks in place to determine if you're using non-expansion content or not.

* Is the player the new race?
* Is the player currently equipped with the new jobs?
* Is the player located in a new expansion-only area?
* What if the player tries to access expansion-only content while using non-expansion content, even if they have it unlocked?

For simplicity it will either deny all characters or simply any characters that have been marked as "expansion accessible". By that I mean if you have any expansion-related content unlocked it will simply lock that character out from being used on a platform that doesn't have the content purchased. However I still think it'll just flat out deny any accounts without the required keys for that platform regardless if they have it for a different one. As for CE items I figure you only need to buy the CE once, standard will suffice on any other platforms considering you can only obtain them one time. Of course that also means you could probably buy the Collector's Edition on say PS4 but the standard edition through say Steam and still get the collector items.