The European leg may be in a flexible bucket perhaps? Seat selection is on a different computer node to ticketing and they don't much talk to each other, so that's not connected. So you should be able to select a seat regardless, so long as you're not using a cheap, inflexible ticket or without status.

Ticketing is indeed queued, if there's a manual change, and prioritised by departure date (with a few ifs and buts for non BA flights). Online changes are automated and near immediate. You are absolutely right to be watching the situation, but you can safely leave it until 48 hours before departure. At which stage you would be in your rights to stay on the line until it is confirmed. However at this point in time you are making the situation worse by calling up, by tying down resources unnecessarily. If your ticketing is prioritised then by definition someone more urgent than you gets de-prioritised.

So check again 48 hours before departure, it's quite normal to be completed a week or so before travel. However don't forget about it completely since there is a known black hole where ticketing gets overlooked. In the old days if you got to the airport unticketed then there was a chance of not flying, but now this less of a risk since now Service Centres other than the issuing Centre can take over bookings, and at least one of them is always open.