Got my SewQuiet 5000 back from Reliable today...
OK...Got my SewQuiet 5000 back from Reliable in Toronto today. The paperwork that was with it says they fixed a wire on the fuse. That explains my problem with it going dead during troubleshooting (new fuses didn't help).
I am going to presume that my original issue of not having enough "oomph" to start the machine on the lowest settings and the sporadic changes in speed on those settings is just the way it is.
In this light, and after reading Reliable's advice to John, I ended up installing a 2 1/2" hardware store pulley with a 5/8 bore (it's just a fraction of a hair larger than the 15mm shaft, but was easily trued up using a thin...very thin...shim opposite the keyway). Now I can bring the "low" speed up into a range where the motor controller behaves when I do small, intricate work and still have plenty of room to raise the RPM so I can speed through the big stuff.
All in all, I'm still satisfied with this servo motor and knowing what I know now, would have just ordered a 30mm pulley along with the stock 90mm pulley wheel from the very beginning.