MT: I can’t answer for how Artscroll or Metzudah translated, but I did give you the explanation as to how to properly translate the word. This should be obvious to anyone who knows Hebrew grammar. And the Gemara in Brachos still works. B’ruriah’s point was that this person was not one who sinned constantly, hence it was still possible to Daven for him to do T’shuvah.
The plural of sins would be Chata’os, in the feminine, as any other similar noun that ends in an Alef (think Kisei). And even if this would be an irregular Chata’im (should such a word exist), for it to be the plural of the word Cheit it would need a Kamatz under the Ches, not a Patach under the Ches with a Dagesh in the Tes, which is how it appears.
Thank you for bringing this up to me though. Artscroll is usually good about these sorts of things. I think I’ll write them a letter, or send them an email, or however one contacts Artscroll.