Well, as for skipping gear changes, this seems to happen more as a function of macro stacking than server-side lag. For example, on the PC version of the game using single lines to denote 16 gear swaps, you can change your gear and use a stat-dependent JA (Chakra) at the same time and get the exact same expected result every time.

eg:
/console exec ChakraGear.txt
/ja "Chakra" <me>

The real gear jumbles start appearing when you have to hit multiple macros. I've been given the impression that each line in a macro is taken sequentially with minor innate delay, rather than simultaneously. This tends to account for the majority of macro-mishaps that occur when two macros are pressed back to back, as pressing a new macro will automatically break/discontinue the previous one.

There is definitely a final check server-side, though, as when you R0 you can change your equipment but your model won't actually blink out. It's my impression that actual item stats are stored server-side, so while the client may be able to tell the server what items you're wearing and what action you're taking, the client won't know what any of those items do until the server processes that information.

Eg: Rather than sending a packet to the server saying "Hey, change my gear to x/x/x/x/x/x/x", and then sending a packet to the server saying "Oh hey, also make me use Chakra", I think it's more likely that the client sends a packet to the server saying "Hey, Greatguardian just used Chakra while wearing x/x/x/x/x/x" followed by "Hey, Greatguardian just attacked the monster while wearing y/y/y/y/y/y/y"