Well, I received some very prompt replies from Kovid in the Calibre forum, along with supporting advice from another member. Rather than post a lengthy reply, here's the thread regarding the issue with attempting smart quote conversions of Mapp & Lucia Omnibus:
https://www.mobileread.com/forums/sho...d.php?t=119518
I'll try to fix the problems manually with Sigil -- no promises due to time constraints -- but if I do, I'll upload it here for everyone's approbation.