I should add: the revised rules are simply heavily edited for clarification; they are not, in fact, new rules.
But I figure I might as well answer the questions directly here, just because I have a free moment right now ;-P
* How are “descriptors” used in play? Do they define or constrain the kinds of adjectives or just serve as fictional anchors?
Descriptors are just fictional anchors. You can have any adjectives you want (or pick up during play). The descriptors themselves are "triggered" by spending an adjective, but then using the descriptor in your description of succeeding at the challenge. So it can't be spent itself -- you have to have an adjective to spend to activate it -- but it allows you to always use your character's "core concept" to describe something, and therefore not have to rely on the adjective spent for your description. It's helpful if your adjectives (especially setbacks) just don't really apply to the challenge you want to succeed at.
* Is there a notion of difficulty? Perhaps a certain challenge may require 3 adjectives to overcome?
Tough challenges are a thing: they require 2 adjectives to succeed. Every combat/physical aggression-type challenge is effectively a tough challenge. You could of course have even more difficult ones that cost 3+ adjectives, but IMHO, that's better served by offering multiple challenges or even tough challenges to multiple characters in order to succeed, or having different "stages" of the challenge so you're presenting more interesting and varied fictional situations. Like a boss monster with multiple forms that require different ways of defeating them, or complex traps that need to be avoided, disabled, resisted, etc., and so on.
* How do monster adjectives get spent or are they again intended as fictional cues?
Enemies usually don't have adjectives, but when/if they do, those are best used as a list of setbacks. A snake wouldn't have "Poisonous" but might instead give you "Sick" because that's the setback you'd get if you choose to fail at a challenge against the snake.
* Do you have an example of combat to share?
Best to just link you directly to the Example of Play for that! Specifically, look at how the player takes the initiative on spending multiple traits in Scene 3: Bandits, because every combat-type challenge is automatically considered a tough challenge.