This is a real curve ball. "If a verb such as essere, diventare, rimanere, with indefinite si, is followed by a predicate adjective, that predicate adjective is plural." (Maiden and Robustelli, p. 125).

Why is it pronti when deve is singular?

I look on it this way: "one must always be ready" refers vaguely to lots of people so the adjective is plural.

What is wrong with "one should always be ready"?

Is it just those three verbs or are there lots of others?