I've been reading up on the subject of CE's splitscreen on PC, and I get it's not possible, but why did gearbox when porting halo decide to remove it?

Another question is that 343 somehow managed to get the Xbox Live coop to support AI in CEA, which, if I'm not mistaken was built off of a build of the PC halo CE, does anyone know how they did it and could similar improvements be made to Custom Edition?

There could be numerous reasons.

1. The technology was shared and not completely owned by GBX therefore sufficient rights were not available.

2. They forgot about it.

1) Because Gearbox did a piss-poor job of porting this game
2) Because Campaign coop online in HCEA/MCC is not using the Halo PC netcode, it's using Halo xbox LAN netcode, which is "lockstep" (look it up). This is why the off-host player experiences significant input delay, because every single aspect of the gameplay is synced in lockstep networking. It's equivalent to playing original Xbox system link over XBC or XLink Kai. It's taking something that was meant for connections at <10 ms latency and using it for connections with much higher latency. It doesn't break but it performs badly off-host.