How does him doing all the hard work make him a kemmlerite? He could simply be a normal wizard using necromancy.
I'm really really sure Corpsetaker isn't SC level. Heck, in GS MOLLY was able to hold her off for a pretty significant amount of time. Remember that mind magic is Corpsetakers speciality and we know that SC wizards are super awesome in their areas of speciality (Eb is a direct combat specialist and he was able to cause a freakin earthquake. The Merlin is a ward specialist and he was able to create a ward that held of the ENTIRE Red Court (If I'm remembering correctly) ). Molly should have been squished in an instant.
We saw that Grevane was charging into battle against Corpsetaker (and she against him) so they clearly thought they could beat each other.
Well, if Cowl is complaining about always doing the hard work, he's also implying some level of cooperation with the other Kemmlerites. Whether or not Cowl was lying is debatable, but you would have to explain why Cowl felt the urge to lie. My copy of DB is not with me at the moment, but doesn't Cowl say that they "play nice" but are always looking to stab each other in the back to be Kemmler's favorite?
Corpsetaker was also confident in taking on (older & scarier) Luccio and the other wardens. Besides, Molly wasn't Corpsetaker's first opponent that night. She had already stalled Harry with shields, taken over Butters, and put the rest of the crew to sleep. A few more moments and Corpsetaker would have steamrolled over Molly too.
At the very least, Corpsetaker and Grevane are at Morgan's level, and in TC Harry said that Eb (the combat specialist) was one of the few that had a good chance of bringing Morgan in.