I'm rereading Cricket Term, and I came across an Ancient Marlow Peter - it's what Geoffrey called his son, after both his father and Kate's, since Nicholas was out until he was definitively dead. So that could have been an additional point in favour of Peter.

In the Autumn Term timeline, Giles would have been born not that long after the end of the war, which could be why he didn't get a Marlow name - they wanted to memorialise Pam's brothers and there'd be plenty more boys for Marlow names, or so Geoff Marlow and his supersperm would have expected. Or maybe also trying to repair the rift with Mme Orly?