Find & fix quote in split paragraphs
In Sigil this expression has been helpful:
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.