My heart goes out to you as you are seeking info and insight into getting pain relief. A situation like yours is like walking on eggshells since you've been diagnosed with RSD and that you are dealing with a severed nerve.

Burntmarshmallow described well the difference between TENS unit and SCS....and there is a BIG difference. The TENS unit evidentally aggrivated the nerves at the surface of your skin, whereas, an SCS is implanted in the dorsal column to BLOCK the pain signals at the nerve root. Your brain doesn't receive the pain signals because the electrical leads transmit a tingling/stimulation instead.
My concern/confusion is regarding the fact that you've got a severed nerve and I'm not clear as to how the SCS would benefit. Also, red flags go up regarding the fact that your pain is spreading. There have been several here who have indicated that the SCS did NOT help in their condition and actually contributed to RSD spreading further.
Jo*mar is helpful by pointing you to the RSD forum, because you'd probably get better feedback that targets your condition.
You are literally riding the fence here and I truly wish there could be a clear answer for you.
Doctors are pushing the SCS like never before and it's a very high-priced commitment, of which these Dr's get a pretty penny from the manufacturers.
But in a case like yours, there are so many variables that I personally would be very leary of rushing into getting this done.
Having the trial procedure would prove crucial in your decision, and you could probably arrange to have the trial for more than 3 days......maybe even a week.
I'm so sorry you've been put in this predicament because of a botched surgery, and now the possibility of RSD spreading to boot.
At the very least, I would go to a high-credential specialist, and even at that I would want to get 2 or 3 other opinions.

Please keep us updated !
I pray for the best possible treatment care plan and that your doctor(s) are looking out for YOUR best interests in the longterm.

Truly Caring
Rae