The code makes even less sense now.
BTW Conversion can only work so many wonders
You have some 'G' in your original.
IMHO Top, Bottom Margins or Padding the preceding or next block is the way to space out paragraphs,headers ...
calibre6 is a Block with a Line height increase which would be OK for the Div, but NOT for the BR which is part of the (non-existent) content within the div. If yo must stay with the Div, simply replace the BR with a <p> </p> (
a space has height)