Miko

exchange

as much as he is able

This phrase might be a reference to the Bishops' Ban of 1599. In that ban, the Archbishop of Canterbury and the Bishop of London signed orders that banned a variety of types of books, including works of satire. If it is indeed such a reference, that coincides well with estimates that the play was written in 1599.

A general purpose word for "offence". In this case, the supposed offence is slander.

Celia means this word in the usual sense of someone who carries a message. In the next line, Touchstone intentionally uses a different sense of "messenger", meaning an officer of the court who has the power to arrest someone. That meaning roughly makes sense (at least enough for a joke) because it was the duke who ordered him to fetch her.