Need parental advice... Booking with a 15month old infant for a 14 hour flight... to buy a seat or not to buy? Feedback welcome

Update: After unanimous wisdom I went ahead and purchased the 3rd seat and will look into seat and harness options. Thank you for knocking some sense in me. Sincerely appreciated everyone.

Im booking a trip to Japan to visit family and Im at an impasse.

My wife and I are bringing our 15th month old person with us and were not sure if we should book 2 Comfort Plus seats and keep our baby in our lap the whole flight baby, or if we should get him a dedicated seat.

We love the idea of getting him a dedicated seat, however after speaking with delta, since he's under 2 he's required to be put into a FAA Approved seat. We have 0 intention to use the seat for the 4 weeks we are at our destination, and lugging a giant carseat around just doesn't sound appealing at all.

Any parents out there that have been in a similar predicament? What did you do and what advice could you offer us?