In an ideal world, most people would sit in their allocated seats because the booking system would give them exactly what they requested, but we don't live in such a world so the current situation, where the requirement to sit in a reserved seat is very, very rarely enforced, seems to be the most pragmatic way to deal with the deficiencies of the system.
Wolfie, I agree with you about charging for the adjustments, but that wouldn't be workable in the first place as many have said. It should be quite sufficient explain, if anyone ever asked, that there is a medical reason for not sitting in the allocated reserved seat, particularly if the system has not been able to provide what was initially requested. Most guards have enough to do, though, without creating a lot more hassle for themselves, though, by trying to enforce the unenforceable.
I'll always sit in my 'proper' seat wherever I can because I think that's the simplest thing to do!