For the first point: not a guideline for destroying the gift, but there is one for creating the gift in Mythic Locations (the fallen temple of Mercury).
CrVi(Co) 75 ritual guideline. Gives the target, who must be human, The Gift. The
target of this ritual must be Individual.
It stands to reason there would be a perdo vim counterpart possibly of around the same level. The spell the guideline comes from is mercurian and stopped working with the changing of the astrological age around the 3rd-5th centuries, but you could possibly use the cult of mercury as a source of inspiration for the breakthrough?
As for other possibilities it seems to me that the obvious solution is if the individual gained the gift through the power of a protogonos then it stands to reason that another powerful spirit might be able to remove it. Perhaps time to look into some theurgy?
A related idea would be to enter a microcosm of the king in the magic realm (this may require the aid of a spirit or daimon to do) and destroy whatever represents his gift in the microcosm, which will be reflected in the real world hopefully by removing the gift. Sort of a mystical "Fantastic Voyage" type thing. I came up with a daimon who can help with just that sort of thing for my 30 Daimons for November project: Fractus. Of course characters would have to find a grimoire with a ritual to summon him, or develop the ritual if they have access to theurgy.
That's the best I can think of off the top of my head, though I feel like there is some way I'm forgetting about to do with hedge magic.