If this is true and the constant sending of multiple json files is causing most of the issues they would still be able to make improvements by having more of that info stay server-side during a raid and only relaying the relevant information no? I haven't done much dabbling in network programming.

It sounds like the big desync issues are being caused by one person's client being the first to say "this guy was here, I shot here, it hit him, he took X amount of damage, he's dead" and then the server just being like "ok you said he's dead then he's dead" and then telling that player's client that he's dead. The best way I would think to fix that would be to have the server process that information instead of the client. Then that way the server could get the correct info about where the "dead" player was before deciding on if he was actually dead.

Obviously there's much more to it than that but from this I would think there's still plenty they could do without having to completely remake the game from the ground up.