In fact, I was trying to figure out the function of "surprising" in the
phrase because I suspected its function may not be that of an adjective,
given that the function of an adjective is to modify a noun, and there
is no noun to modify. A gerund is a word that is created with a verb but
functions as a noun. To me, that's a more satisfying description of "the
least surprising possible" than is the assertion that "surprising"
functions as an adjective in that phrase.
However, I'm open to any arguments supporting an alternative view, given
that I do not claim to be a grammar expert. (I think that I'm in good
company. Even some professional editors I know don't claim to be grammar
experts and will point to others who they identify as "more expert in
grammar" when asked a particularly problematic question. These days it
has become a rare obsession to be one who always looks for grammar rules
to explain natural language phenomena.)
I may have entirely wandered off of the point of the original question
by now, but my adjective preoccupation was only triggered in trying to
accommodate the "aggettivo" element of the "il meno <aggettivo>
possibile" specification you supplied.
In searching for opinions regarding adjectives with no nouns, I happened
upon the article cited below that I only cite because I think it might
be of some interest to you.
https://guinlist.wordpress.com/tag/adjective-without-noun/
The site author claims to be a specialist in English for academic and
professional purposes, and he says "The blog particularly seeks to
clarify advanced topics in English grammar, vocabulary and pronunciation
that give the greatest problems to speakers of other languages when they
are involved in academic and professional communication."