Thanks, Janey & John.
Just after I posted, I continued to research and found an article suggesting using a hair
dryer to warm up the inner parts of the machine (this is after I cleaned - not much needed - and oiled the parts. I tried that, and moved the dial between stitch selections slowly, over and over, and then tried them all again. Voila! All of the stretch stitches stitched perfectly in the forward direction!
The only issue that remains - and I do not want to run the machine until I find out more about this - is that the hand wheel is very stiff to turn. The innards seem to be moving nicely (when viewing them from the bottom of the machine, or inside with the top cover off). So I am concerned that the motor itself may be frozen up to some extent, and I don't want to burn it out by running it. I am assuming it is pre-lubed, since it is not like the Featherweight or Singer 301 which have a port for lubing the motor. Would it be safe to heat up the motor itself with the hair
dryer to see if that would loosen it up? The belt looks like it is brand new, and probably has never been replaced (the machine was hardly used), so I don't think it was installed too tight. Anything else I should look for or try?