Two questions on the Eurostar.

1) How do seat assignments work? Very often I try to change seats online, I only see a few seats available, and then the carriage is half empty. This morning I had the entire carriage to myself, after the online seat map last night showed me exactly two seats to choose from. Surely not all of these seats are no shows? Could it be that there are contingencies for different countries of purchase, i.e. people buying in France only have access to certain seats and people buying in the UK have access to other seats?

Also, do Carte Blanche cardholders get preferred seating for instance for the solo seats?

2) Is there any way around that (in my mind quite silly) restriction that tickets booked less than 48 hours before departure have to be picked up at the station? Why is that anyway?