Your situation is quite different than recently constructed sidewalks being unsafe to navigate by people with disabilities or the elderly, and lacking such basic features as curb cuts. Don't be so black and white.
I think you are exaggerating the issue though.
Ideally, an intelligent and flexible application of the ADA rules could be envisioned.
(Ideally we would prioritize research into regeneration and fix the real problem.)
One of my friends was born quadriplegic and literally cannot live without help. He can't go to the bathroom without help. He can't pick his nose. Traveling around Seattle with him made me realize the fact that accessibility makes us a worthwhile civilization. We care, and spend to show it.
Consider: If you spend ten thousand dollars on accessibility for your barn, and no one ever uses it, that's a good thing.
Pick your nose and count your blessings.
As for "flexible" and "intelligent" govt. regulation., I'll admit I chuckled when I typed that in. We'll be regrowing limbs long before that, I'll wager.
I'm not sure how to remedy the situation until then. It sucks that you have this extra burden just to use your own barn.
edit: Are you retrofitted your entire house to be fully ADA compliant? If not, why should the parent be required to?
If it's 100% incorrect as you claim, surely you can provide evidence supporting your claim?
A place zoned residential can fall under ADA compliance if the use is commercial. Exactly as that link states. So the first part of your statement "Houses are not commercial establishments" is correct for zoning classification, but you fail on the second half "as such are not covered under the ADA." - a residence operating as a business can be under ADA.
My comment "A residential structure can still be required to be ADA compliant dependent on use." - is 100% correct based on the very evidence you supplied.
At this point, why dont you just accept that this is a very complex topic and one that just maybe I've spent more time navigating than you.