I'm not being sceptical, I'm just trying to honestly set out the facts so readers can choose whether or not to be sceptical themselves

It's not my place to make choices for people that could have dire consequences, however slight the risk may be. The original intention was that this topic would explain the possible interpretation of the rules and gather together the official information; it wasn't supposed to carry on the discussion, we had lots of the same before I wrote it.
The IVA regulations aren't really the issue, it is what the DVLA will allow you to do to your car before they consider it's no longer the same vehicle and ask for their V5 back. If you like, you can also take in to account, the likelihood that they'll ever be aware of, or bother themselves with considering your car at all.
Parliament made the regulations, and ultimately it'd be down to a court of law, or equivalent authority that has the job of enforcing them, to decide what they mean. Until an enforcement is made what anyone says, including DVLA & VOSA employees, is just opinion. That is why they are loathed to give you straight answers.
I don't think anyone is disputing that there is a 10 year exemption; ten year old cars that continue to be used on the road may well not pass parts of the IVA - so you could argue old cars, that are newly registered for our roads, will be no more dangerous than the ones that are already registered. However, the whole purpose of the IVA test is to assess individually built "one-offs". Few, if any, will be whittled entirely from new materials, so there has to be a point where a home built car ceases to be the donor(s) and becomes a new vehicle in its own right. The DVLA have to decide when that is, as they issue registrations, hence they devised their points system.
Ten year old cars will still have the same pattern of major components (at least to score enough points with the DVLA) that they had 10 years ago. Those that don't aren't ten years old, even if some (or even all) of their components are.
You can't make an antique telly cabinet out of a 100 year old sideboard.