You seem to have hit the problem on the head. Playing around with the toggle switch didn't appear to help any. Matter of fact I'm only getting the engine to run when it's in one position. Can you provide any tips on how you added a roller to the tender. Looking at the bottom of it, it doesn't appear to be any each way to do this. Also, I'm assuming the toggle you're speaking of is the three position switch at the rear of the locomotive. When I dug this out of storage it was set in the center position. I tried moving it both right and left but I'm unsure which position is the forward position looking at it from the bottom. Can you confirm. Thanks.
Before, adding a roller to the tender, you should ensure the loco itself is operating correctly on regular track. You should remove the motor/e-unit assembly by removing one screw on top of the boiler and two screws underneath. That will facilitate maintenance & repair.
As Train Larry mentioned, the E-unit lever is two position only. All the way left and all the right. T-Man provided a good picture of that lever.
Note Larry's post on the the lever's "feel" when moving it. If it's loose, that has to be corrected (easy). If it's tight, the next item I'd check would be the E-unit's electrical contacts (the "drum" and "fingers"). Are they tarnished or green? You can clean them by repeatedly spraying a contact cleaner (CRC @ HD, Goo-Gone, etc.) and cycling the E-unit (e.g., spray the drum and fingers, put it on the track and toggle transformer power several times, repeat, repeat, etc.). That may clean the electrical contacts to restore proper operation. If that fails, you need to service the E-unit (more involved).
If you have both the lever and E-unit contacts working well, then you should be able to lock the Loco in forward to solve the crossover issue. At that point, if you want the F-N-R control while also having a crossover, you need to add the tender roller to mitigate the momentary loss of power over the crossover cycling the E-unit into neutral.