Welcome to NeuroTalk:

Looks like you figured it out well. That is the OTC equivalent to Metanx. Except it is the lower dose of methylfolate, which is okay. MetaFolin is hard to find, because for a while Merck was not selling it to OTC outlets. Now that has changed.

Were you ever tested for B12 levels? Do you know the actual results?

Metanx will work if your issues are based on low B12 levels, which lead to nerve damage over time.

The fact that your PN didn't progress to other areas over a long period, and remains the same implies perhaps something
else may be going on.

You could just start with the B12 and not use the other two in the beginning. This would save you some money. The B12 is the most useful I think for most people. Those with DNA inability to methylate folic acid would need the methylfolate in the Metanx as well. But at this time you don't know if you are one of those folks.

Did you not feel any numbness in the feet at all before your fusion? If you can think back to when your feet started up, what was going on with you then? Drugs? Statins? Vaccines?
exposure to toxins? illnesses like severe flu or infections?
Chemo? Do you use acid blocking drugs often or every day?
About 30 percent of people lose stomach acid with age, and this is called achlorhydria. When this happens, the B12 in food is not broken down for absorption, and low levels result. It can be due to aging, autoimmune destruction of the parietal cells that make intrinsic factor, or due to acid blocking drug use.
So in older patients low B12 can become quite common.