Truth is the late Queen always had a standing invitation for Hazno and his family to visit her. In fact, one of the things I cannot forgive Hazno is that on her last summer he was in Europe - days before the Queen died - and refused to visit her (but then had a tantrum because he couldn't take the ILBW to Balmoral after she had passed).
That said, this was all before the Netflix reality show, before the book and before the BBC interview where Hazno claimed his family wanted him dead.
My point is, if the Harkles were a bit smarter and less transactional in all they do, they could have kept a family relationship without being working royals.
But as I said, it was all before. Hazno and the ILBW have proven over and over that they are snakes who regard the royals as a piggy bank, the titles as a for profit brand, and the monarchy an asset to elevate their worth (while using it to victimise themselves). And this is why any approach now is unwise for the Royals, is a dumb weak move from the King.
If KC is worried about the institution he is head of, the line between him and his traitor son must be clear: no contact, no support. Sure is hard. That's why the top job is not for everybody: it's for the ones with the strength and sense of duty to fulfil the job description. If Charles is not up to it, it is time for the next in line.