I have a happy ending to report. Strida Europe / Vanmoof have sent me a replacement – the SX – and it is functioning well. It has the new EN-style frame, which I am pleased to say does give me an additional sense of confidence, and feels nice and firm. Thank you to Vanmoof for the prompt and fair customer service; special thanks to Joffrey!
As for the SX itself: I am very pleased with the handling of the bike so far. I didn’t really think before that 18" wheels would make much of a difference, but it does seem zippier than the LT. My other concern was that the wheels would be too big for my commute, during which I stow the Strida on the luggage rack of a train, but this was not a problem. My recommendation would be go for the 18" wheels unless you have a specific need for smaller wheels!
Coincidentally, my first proper ride with the SX was on the day when we had our first snowfall here. It was mushy, icy stuff, which has often given me problems in the past. I had been planning on riding my Brompton instead, but it had a flat. 
So I gave the SX a chance, and I found that it felt much more stable in the snow, even with thinner tires! (It came equipped with 18" Innova 85 PSI tires.) Also, I didn’t get any ice accumulating on the belt or the rear freewheel, or getting stuck between the freewheel and the snubber. I think the snubber position was already improved on the LT. I wonder if this was just a happy coincidence, or whether Ming was gathering data from riders in snowy climates. 