Sometimes when a thought comes from someone else it resonates with me. This one is my thought.
Treating B12 deficency could be better expressed as treating the damage caused by having been B12 deficient.
Severity of symptoms could be better expressed as the amount of damage caused by B12 deficiency. It would still be subjective but it might help as an explanation why EOD is thought to eventually work and does for some and not others.
B12 deficiency is easily corrected, loading then maintenance. That in all countries which use different protocols and forms of B12 is the accepted theory. That being done you will either heal or not. Problem solved.
The EOD protocol is the most robust that is official that I have come across. The either you will heal in time or not is still the accepted outcome. Some, myself included, exceed this protocol.
I personally do not accept the concept of you will heal in time or not. I accept both that may not be true and that it may be. I personally endeavor to give myself the best possible chance and not simply wait any longer.