That makes sense. No, it wouldn't be the Doogie Howser effect, because outside of a young body, he'd be inside Harry's head and conducting that body. However you maybe on to something,switching bodies can cause problems as Luccio discovered, she no longer could make warden swords. Though that might be because she was dumped into a non-wizard body. The Corpsetaker on the other hand doesn't seem to have that problem with each switch she makes.
I just did a reread of Ghost Story. Corpsetaker wanted to possess either Mortimer whose skills were already undead leaning or Molly whose White Council quality. Also in Luccio's short story Kemmler was weak enough to be held by a Venatore circle jail. I think that suggests he was between bodies of enough talent at the time.
NecroBob was also one of Kemmler's creatures.
But then he could have just not trained Harry and Elaine beyond the basics. No need to make Harry an enforcer if he's going to body switch him. Actually, just be a semi-decent parent to them so the heel turn is more effective later.
Even at his height Kemmler seemed to like able minions not dissimilar to Drakul.
Besides Harry and Elaine are Starborn. Which I think is what Justin/Kemmler wanted them for. He himself isn't one even if he were to inhabite one of them. He needed loyal Starborn servants. Something he couldn't do or be himself.