You really are in a pickle! You have my complete sympathy on making such a difficult decision. How long do you still have on your cobra? Is this a decision you have to make immediately or do you have a little room for mulling it over? Which of the doctors would do the surgery if you had it?
It sounds to me that the nerve issues are getting to the point you want them dealt with, but don't want to take too big a risk. Speaking as one who has mild numbness in both feet, once it has been there a while it doesn't get better. If you talk with the neurologist who ran your tests they might be able to tell you how fast this is likely to progress and what your risks are if you delay surgery. Sometimes there is a window of opportunity on these things for a best recovery. Hopefully your neurologist will be the neutral party who helps you sort things out.
As for the piriformis- I have had the piriformis and as a result my whole butt in spasms for weeks multiple times. I have found that my chiropractor has helped and so has regular use of a hot rice pack to loosen the muscles at home. Make it as hot as you can stand and keep it on as long as it is warm. When it is bad I do this watching tv and sit on the pack, then repeat. Hope it helps.
For myself, I think I would err on the side of informed hope if I made this decision. If things continue to get worse, the foot drop is a very strong possibility. While it is possible with surgery, it is at least to two of your doctors less risk than doing nothing. Hit every website, talk to everyone you know, learn from this website (but remember this is highly skewed to people who had problems), ask all your questions, then pray about it.
God bless you as you make your decision, and let us know how things go.