I don’t know enough about the circumstances surrounding the Harkle kids well enough to comment on all your post, but three points I would add:
1. Parliament has conferred backdated legitimacy in the past, and also taken it away. I mentioned Henry VIII as the best known example whereby Mary Tudor was legitimised in LoS, delegitimised and then legitimised again, as was Elizabeth Tudor. There’s also the example of Henry II and his four sons although the Parliament pre John was so different from what we have today that I don’t think it makes a good comparison.
2. I see the phrase “born of the body” a lot in these threads and was just wondering where that came from? The 17 century legal wording was “heir of the body” and it was debated in the Lords in the 2013 Succession to the Crown Bill in which Lord True pointed out that this would exclude children of a Same sex marriage or born of a surrogate and should be amended to genetic heir. That’s an interesting debate to read for anyone interested in this issue and can be found here:
https://hansard.parliament.uk/lords/2013-03-13/debates/13031351000661/SuccessionToTheCrownBill
3. If Lady C is correct then I reckon KC3 has been consulting the Law Lords and seeing if there’s a way around whatever issues exist with the Harkle kids (of which you are clearly better informed than I am!)
I find it all very fascinating tbh.