It’s not correct that you can’t add an infant to a J redemption ticket, unless they changed their policy very recently. I booked a redemption in J with a lap infant a couple of weeks ago online. Hardest part was the payment process, but that’s a different topic here.

I would try a few more times with different agents. If that doesn’t work, cancelling and booking online through the app (not the website) seems the best option. But as mentioned above there is a serious risk you won’t be able to book the exact same seats.