In Sigil this expression has been helpful:
(“[^”\r\n]*)</p>\s+<p class="calibre.">
Replace with (has a trailing space): \1
This indentifies paragraphs where a opening smart quote is not matched with a closing smart quote and joins that paragraph with the next one. Its not fool proof, but saves a lot of time.
I use calibre conversion to switch straight quotes to smart quotes. Its under "Look Feel", check by "smarten punctuation". Easier to fix its mistakes than to find and fix 'em all.
Good Luck!