OK, I will leave the thread after this because I can see your mind is made up and I don't want to keep digging, although I agree with the suggestion above that if this seat needs to be used because of a disability, to explain about it in that way.
I just wanted to add one more piece of information which you may or may not be aware of. This car seat conforms to R44.03, which is still legal, but is very outdated. The UK manual for it (which was hard to find owing to proliferation of the US manual) boasts that it meets the "revised" standard suggesting that the approval happened somewhere around the adoption of revision 3, which was 1995. The Husky (which is not identical) has not been produced in the US since 2005. While some of the newer regulations pertain to aspects such as avoiding misuse, and it sounds to me like you're the kind of person who would have read the manual back to front and made damned sure not to misuse the seat, some of them also apply to things like having lower tolerance for forward rotation or better side impact protection, things which actually make a measurable difference in the outcome if there was to be a crash.
The approvals for special needs restraints are slightly more relaxed than the approvals for car seats for general use, because the understanding is that sometimes there is a trade off between having someone restrained at all in an accident vs maintaining a high standard which is impossibly hard to reach when the occupant has a higher body weight than is usually standard for this kind of restraint. For this reason, special needs restraints need to have a disclaimer somewhere on the manual. This seat has this wording on the front cover, which is actually the same wording which is given in the most up to date version of the safety standard:
This “Special Needs Restraint” is designed to give extra support to children who have difficulty in sitting correctly in conventional seats. Always consult your doctor to make sure that this restraint system is suitable for your child.
The UK law is not very clear about whether using a special needs restraint without medical advice is permissible, it just says that you need to use a child restraint conforming to R44.03, R44.04 or R129. That said, there is a clear benefit in the case of disability where a child either lacks the physical strength or cognitive awareness to use a standard booster seat properly, which likely outweighs the risks of the seat design being 30 years old (it will have been manufactured much more recently than this obviously). If the child does not have such a disability, then the benefits probably don't outweigh the risks any more, which is why this statement is printed on those seats in order to discuss the individual situation with a medical professional (who probably doesn't have any particular expertise in car seats, but should be able to assess a child's ability to understand or physically support themselves).