My simple view is that F and J has bassinet positions for the purpose of parents travelling with their small children, and is infants were not supposed to be in premium cabins, there would be no bassinet positions. All problems I have experienced with kids in lounges and planes have basically been parent induced.

Expecting as a certainty that your child will have a regular and stable sleeping pattern at 5 months is slightly off, your child may, but alps just as likely may not have a stable sleeping pattern. And sleeping in a foreign environment may very well cause a lot of unrest. I am not writing this to discourage you from booking, just saying you will only know by actual experience.

On a side note, I wonder how easy or difficult it will be to get redemptions in connection with the Olympics, even assuming JAL releases the normal level inventory, you are hardly the only person wanting to book. And JMB members now get a head start as the availability opens earlier for JAL' s own members.