>>But quis is "who", as in "Quis custodiet ipsos custodes" (who will guard the guardians).
>
>As I said, my Latin is VERY rusty. I understood that word to be What, Who, Which.
>
>Again, demonstrandum is the infinitive of the verb?

That's the gerund, i.e. sort of a passive verb adjective with the "to be {verb here}ed" meaning.