So I just played a few skirmish games to test my earlier report about the "endless CP bug", and it seems that for at least all British/Commonwealth doctrines, one can still earn command points even when all abilities in the command tree have already been unlocked. This was supposed to be fixed in 1.7.0.0, but it looks like it still persists in the current version.
I also noticed that the maximum number of extra CPs for each doctrine is different. With Royal Engineers Support and Royal Artillery Support, you can get up to two CPs. With Royal Commandos Support, you can have up to five. The number of CPs will simply roll back to that number no matter how much experience you gain.
Here are a few screenshots showing the bug...
Royal Marines Commandos + Royal Engineers Support:
Commonwealth + Royal Commandos Support:
Commonwealth + Royal Artillery Support: