The "let" is verb or past participle ?
Does the sentence have two verbs ?
"Let alone" is a phrase used to say that something is even more difficult to do than another thing. (dictionary) "Let" is a verb in imperative form, but the meaning is from the idiom. Don't try to understand it word-by-word.
It means that it is very difficult to replenish a payroll, and it is even harder to expand a payroll.
I count four verbs: "is", "replenish", "let" and "expand". The main verb is "is"