A couple of quick questions related to "Messrs"

Solution 1:

It is pronounced Mess-ers derived from the French plural messieurs, while Mr comes from master. Its use is correct but fairly uncommon: Mr Smith and Mr Jones is more usual than Messrs Smith and Jones; Misters Smith and Jones is extremely rare while Mrs Smith and Jones would suggest a sex-change.

Solution 2:

Does that mean that "Misters" is not a word?

The plural of Mister is used; you can find sentences like the following:

The gold medals they won at earlier races aren't the only thing that Misters Hedrick and Cheek have in common.

Is "Messrs" a word that is more commonly used in England than the United States?

The NOAD describes Messrs. as dated or chiefly British.