After all that, I think it was a simpler solution just to fix the parser’s unhelpful .noverb grammar behaviour, as in this previously-mentioned thread!
EDIT: although that woudn’t prevent the player foolishly or inadvertently naming their spell the same as a single-word command such as ‘jump’- in which case they would still have to use ‘cast jump’ rather than just ‘jump’ to get it to fire.
EDIT2: either way, the minor issue will remain with both approaches that typing a ‘verbless’ command in response to a disambiguation question will incorrectly not be recognised as a new command (rather than an answer to the disambiguation question), and so will fail- probably with the response ‘I did not understand that sentence’- see also here.