Not working together but coordinating at arm's length. This happens a lot, even among sides that might appear to be engaged in a 'PR war'. That's my professional opinion; I could be wrong and other, doubtless better, professional opinions are available.
The RF learned after their bitter experiences with Diana that it's better to keep lines of contact with a volatile member open, than shut them out completely. Not an embrace, but an arm's length handhold. This is the strategy they've adopted with Andrew.
Ultimately, if you're a family firm and one of your members has gone off the reservation (to use a colonialist phrase) and might be a bit unpredictable (and crucially, knows where all the skeletons are), your firm will be safer in the long term with that member in the tent pissing out, than outside the tent pissing in. Even if you relegate them to a far corner of the tent and don't talk to them very much.
There's a difference between viewing these things through a moral/emotional lens ("I do wonder how the RF could ever trust H&M again") and viewing them dispassionately through a tactical lens. The RF hasn't survived for this long by letting emotion drive strategy and tactics. And you don't have to trust someone to keep them fairly close (in fact, not trusting them is a good reason FOR keeping them close).
Extending a hand of friendship by attending Invictus 27 would be BRILLIANT PR for W&M. They immediately look forgiving, wise and statesmanlike; all the things you want in a future King.
It might also help resuscitate Invictus after a very, very fallow couple of years.