Polly - re Rail Boutique Europe in Geneva.
It opens at 8.30am. It looks like a travel agents, on the corner of the main square where the station is situated.
My sister had to pick tickets up from there this morning (same route as you). Station staff twice put us in the wrong queue inside the station. By the time we found Rail Boutique Europe, the queue outside it was an hour long. (It's understaffed, because of the train strike). So staff told us to just get on the train with the email print-out.
As it happened, my sister made it to Paris, then on to Calais today, without seeing one ticket inspector - nobody asked to see her tickets, let alone email print-out.
So make sure you get there in VERY good time, and don't miss your train if you can't get your tickets in time - just jump on board! Good luck.
PS. My sister just sent me a text saying 'there'll be bluebirds over...' with a pic of the white cliffs of Dover. She made it! Plenty of ferries, and availability in Calais, despite hectic crowds at the station, and having to foot it across town. No need to buy a bicycle, only cost her £100 from Geneva. Now for a train to Aberdeen though...