I agree that its good not to discount anything, but with the evidence there I can't see any way in which that statement can be interpreted otherwise. Any change to make it not mean a castor (noun) who is non-royal (adjective) would require retconjuration. Its not a grey area, grammatically. All those examples of subtly-worded clues all worked because the 'real' meaning of the mystery was logically referenced when you knew where to look. 'into the hands of a non-royal caster' (and its corollary of royal caster). 'Caster' is being used in the phrases as a noun not a description (adjective), what we have effectively is:
"[Slately]__[put]_[the]___{____________[Royal Scepter]____[of]_________[Jetstone]_____}_[in]
[pronoun] [verb] [article] {
noun phrase: [noun (accusative)] [preposition] [noun (genitive)]} [preposition]
"[the]__{_____________[hands]_______[of]________[a]_____[non-Royal][caster]_______}..."
[article] {
noun phrase: [noun (dative)] [preposition] [article] [adjective] [noun (genitive)]}
Which is to say the sceptre was put in the hands
of (indicates a possessive (genitive) noun to follow) [b/]a[/b] (article implying one specific noun of a class)
non-royal caster (which therefore has to be: a noun, not a description; in the genitive; in the singular. 'Non-royal' cannot be a noun it is adjectival, ('non royal' could), and 'caster' cannot be descriptive (adjectival) in this sense as there would be no noun being modified. Its an absolute statement, and without scope for ambiguity.
To change that absolute statement into meaning that no caster can be royal is as much a rules violation as making a unit with a maximum Move of five move six hexes - or a dead twoll with the 'regeneration' special be un-decryptable and unable to recover from a fall :p
I'm happy to agree to disagree on this, of course (sadly, I'm also happy to parse grammar, argue philology and generally de-construct the English language

)!! And time will undoubtedly tell what was meant; or be retconjured to suit.