1) What does the raised comma represent?
It's an apostrophe (pronounced apostrofi) which represents missing letters, as in it'll (it will). The missing letters in 'twixt are b and e - betwixt. This is an old-fashioned word meaning between.
2) Do we need to put it in when we quote the English saying?
Yes, if you want to be correct. Apostrophes are not normally left out.