I am having belt issues as well with my New Home/Janome and did exactly what you did with the same result. Have yet to return the 3 I got from Jenny....
My local Sm place was not near as helpful as yours. To determine the belt size, and whether they even have one in stock, they were going to send my machine in to the part supply location, where, I'm sure, they look at data and go find the darn belt if they have it. What would be the extra charge for this service, I wonder.....
I thought the run-around I'm getting was particular for Janome's customer "service", but I guess not. So I'm back to using the very tight belt the machine came with. The stretch belt I got doesn't work either, since it interferes with the bobbin winder.
Sorry I'm no help, we're in the same boat - but at least you're not alone ;-)