I think your regex is fine...
calibre reformats the comments, putting them into a final div IIRC, and if you have multi-paragraph comments those need to be divided up
somehow.

Paragraph == anytime there is a line-break.
In HTML mode line-breaks are collapsed into whitespace and paragraphs respect the <p> tags, but after nuking the tags if you have line-breaks calibre will do it's best to recreate a basic structure, and probably inserted hard line-breaks where it thought (

) you wanted them.