I never said Kemmler was Starborn, it would be a reason if it is required for a successful Darkhallow that he DIDN’T do it himself, and withheld this knowledge from the heirs.
It may be though that you can do it if you body swap a Starborn, but you have to have one available to Bodyswap and the new batch wouldn’t be born for nearly a decade after his last reported death. The only known surviving Starborn from a previous sequence is Drakul. Good luck trying to steal his body, especially with the White Council baying for your blood
Kemmler could have been waiting for someone like Harry to be born, when he got killed again. Indeed Justin could have been waiting for someone just like Harry for Kemmler to bodyswap into, he had Bob and the Word, and you couldn't be sure Kemmler was dead as he had been killed seven times previously. Maybe Justin was waiting every day for Kemmler to contact him, or was on the verge or resurrecting him in Harry’s body when he started to enthrall Elaine, and got killed himself by Harry. We don’t know much about Justin’s motivations, but controlling two thug wizards wouldn’t be a challenge to the White Council, but being High Priest to a resurrected Dark God Kemmler with Elaine as High Priestess might appeal more. Maybe Justin was the true heir of Kemmler.
It may be that Harry killed not only Justin, but also Kemmlers last chance at resurrection with Justin. Kemmler always had an escape plan, from what we have seen this time it might have been Justin, being a hidden Kemmlerite amongst the Wardens.
Isn’t there a WOJ that Elaine was Starborn as well?
Journal indicates that several factions in the White Council knew of Harry’s status as a Starborn, at the very least Morgan, Eb, and Justin.