I've read the thread (originally and now) and don't understand logically what the fourth option is... Could you explain it, maybe a different way, for me?

Okay. I'll do what I can. First, let's be clear what I mean when I use the term "standard." In this context, we're talking about the bar that is met. When a manager evaluates your work, they are doing so against a standard which is expected to be met or exceeded.
What TrueJim has created is an either/or situation. You either have high standards or you have low standards. The third option is somewhat illusory, because it's essentially no standards. When you create a unique set of standards for each person, it's no longer a standard at all.
What I am suggesting is that high or low standards is irrelevant. I'm suggesting that consistency is the key, whatever the standard. One can have high standards that are not consistently enforced. One can also have low standards that are inconsistently enforced. Further, the very use of the terms "high" and "low" are subjective. What is a high standard? For you, that may be something completely different than for me.
As a quick aside, when you frame the situation up as TrueJim has done, you are creating a red herring (and also a false dilemma). For what that's worth, it's technically illogical for at least two reasons.
Getting back to the point, though, the key isn't to worry at all about what the standards are elsewhere. Rather, what is the standard in this system, in this school? And how do we measure performance against this standard in a way that is consistent and equitable? Where the standards are arbitrary, they become meaningless. Where they are consistently applied, they are meaningful.
If your school has one single standard for earning a black belt, and that's time on the mats, great. You have a clear standard that can be communicated to all and managed consistently. X hours on the mat = Black Belt. Whether you are in a wheelchair or not, deaf, blind, whatever.
If the standard is that the blue belts should be able to consistently perform well against white belts, and so on, then that is an okay standard. It's clear and able to be managed consistently.
The confusion comes about when people use the term "standard" and "measure" interchangeably. The standard is fixed. How you measure performance against the standard isn't necessarily so, and that's where you can both maintain the integrity of your standards while accommodating disabilities in creative and positive ways.