"Be assured, my dear Sir, that Mrs. Collins and myself sincerely sympathise with you, and all your respectable family, in your present distress, which must be of the bitterest kind, because proceeding from a cause which no time can remove."

I have always thought that to have the correct grammar you would remove the first person to get the proper tense (eg. Mrs. Collins and I)...

Language and usage was different back then, so perhaps this was just the proper way of speaking, or is indictive of Mr. C's pompous style (to his ears, perhaps it sounds more formal), or (in my opinion) means to show his lack of education. Ch 15 says "MR. COLLINS was not a sensible man, and the deficiency of nature had been but little assisted by education or society."

In an earlier chapter, Lydia says something about "Kitty and me" which I also gathered was to show her (relative) lack of education.