There are very few bassinets left on A319s now. I'm tempted to say "none", there may be one or two still left, but I doubt it. In a sense the easiest there is for the infant to be in CE and give up the middle seat for that child. From LGW this is easier than LHR since the middle table could be easier to manipulate.

As for intermingling, I think you'd have to be sensitive to other passengers there. If the flight isn't very full then if the CSM is OK with it I can't see a problem. But if the flight is full, I think the other passengers could become annoyed if there is a lot of to and fro through the CE curtain. You could perhaps give up your seats in CE and all sit together in ET. Formally this can be done by changing RFS bookings, so subject to availability. Informally you could ask the CSM to "upgrade" a couple near your other family members and you then take those seats. It is a short flight really, I'd imagine your real family time starts when you get to Austria.