Harry will always be Royal as he’s a blood Prince. Even if their Sussex titles were removed, he’d still be Royal.The couple are duke and duchess at Her Majesty's pleasure. She can give the duchy to someone else, at which time the couple are no longer "Sussex". I would suggest that Her Majesty still has quite a bit of power over the two unless they plan to simply live quietly on their existing investments.
I can register a company as Sussex Royal Inc as well, assuming it's not already taken. I'm not the Duke of Sussex or a prince though, so I would have difficulty getting people to part with their cash for the privilege of wearing my designer label.
The point of the "Sussex Royal" brand is to give an aura of glamour to whatever fashion items they chose to apply their label to. If they are allowed to rebrand as "Sussex Fashion" or "Sussex Styles" or something like that, then the glamour is still there. If they no longer end up as a real duke and duchess with an unspoken direct link to the top though, then what is the "brand value" they offer?
I can't say that I will be overly surprised if it is indeed true that they won't be allowed to use the "Royal" label for commercial purposes.
My point is, how can they be stopped? If they choose to ignore the Queen’s direction, how would that be enforced? In a US Court? I can’t see there’s any way the Queen can stop H&M from calling their business Sussex Royal. If they do, they’ll obviously cause more friction within the family.
I think he could put two fingers up to granny and just do it.