I know boxing on any specific server is prohibited, but...
 

...what about playing simultaneously on different servers? I.E can I be camping a mob on Blue, open up another client and play on Red while I'm doing that, under the same account?

Yes I believe that is allowed. Just do not log into 2 accounts on the same server unless you have an IP exemption for a guest or house mate.

I was playing on red while camping the Priest of Najena on green. I'm fairly sure this is expressly allowed.

Yep, doing this many times, playing green and blue at the same time, although on two different computers. Same account. On one screen (server) meditating, on another attacking or waiting on a bot ride over OOT etc...

Actually, you could legally do this with all three servers

Cool, thank you. I'm camping a guard in Paineel that only respawns every 24 minutes on my 40 Paladin, as it's the safest method of solo leveling for me at the moment (and roughly 10pp a kill), so I can spend that downtime playing a character on Red between pops.