We have contradicting WOJ Elaine is also Starborn, and there should be more, even with Wizards low reproduction rate (we are pointing a finger at you Mr Ramirez) there has been a recent population explosion e.g. the baby wardens, so there must have been more than just Harry and Elaine in that cohort. Could Kravitz and some of the other Warlocks around Harry’s age also have been Starborn?
Another thought, it is assumed Cowl made the Hexenwulf belts, but what if instead he got them from the Swartalves? It’s very much within their bailiwick of magic. The Red Court went to the Fomor for their bomb, a biological weapon, not Cowl, so why not? There might be a long running tie up or membership in the Black Circle/Black Council and the Swartalves, and Etri’s death may have been the pay off. If so missed twice.