This will probably be my last update.
I (finally) got a call from my neuro on Friday: she concurred with my reading of the EMG report. The report rules out ALS at present and suggests a possible radiculopathy. There is no laboratory support for an MS diagnosis because the NCS showed slowing of conduction in the peripheral nerves - but that doesn't mean there is no central slowing, so that possibility is still on the table. But there are no further tests to do to either rule it in or out.
The bottom line is she doesn't know what to make of my results, other than that it's not ALS. Apparently i have a (mild, she said) lumbosacral radiculopathy, and that could account for most of my symptoms... but absolutely NOT the upper motor neuron signs, which point to something going on in the brain or, more likely, the cord. (She was quite surprised that the EMG neuro had said that S1 radiculopathy could cause clonus --- "no way!" were her words.)
She said that I could wait for and schedule the appointment with neuromuscular if I want since they might pick up something she missed. I'm still waiting for the callback but I'm inclined not to follow through there at this point because all of the diagnostic tests done at the hospital are charged at full rate, of which I pay a 20% co-insurance. I'm already on the hook for several $1000s and can't afford too many more tests, especially repeat tests which, she said, they will almost certainly want. She agreed that was reasonable and suggested that the best course then is to watch me over time and see if there is any change/progression.
So I wish I could wrap things up nicely and say not only, no ALS, but it was this, or that, and see everyone even when there are UMN signs and something clearly wrong, it isn't necessarily ALS. But I can't do that. It still could be UMN disease but unless it spreads to a new area, there is no reason to suspect that at the moment. Apparently whatever it is might become clear with time, or it might not. As my neuro said, sometimes neurological problems don't have clear diagnoses. I'm concerned but not terribly worried, as long as I can still do all the things I've always been able to. If that changes, or a future exam shows anything new or concerning, then I'll of course feel differently!
Best of luck to everyone here, and thanks again for all of the helpful information.