The only reason I pointed it out is because we had a similar situation too - we brought our little one home when she was almost 16 months old. The plan was to have both DH and I return with her - we faced exactly the same situation as you did - and decided we'll book one economy and one of us uses it for half the flight and switches.

Never happened that way as I ended up bringing her home alone with me anyway.

You comment "The infant is ajust along for the ride" made me chuckle - believe me, very soon, he/she will take you on a ride of your life where you'll want to give her/him to your wife in BF and relax in economy yourself