HTML Tidy ...
 

Is it possible to expose the options for HTML Tidy, maybe in an INI file?

No, it only uses part of HTML Tidy anyway AFAIK and most people here turn it off totally or set it to pretty print only.

Furthermore, next generations of Sigil will no longer employ Tidy. It will instead be based around an html5 parser using the original html5 parsing spec.

Tidy was never meant for HTML5. Even the HTML5 fork never got it working properly. Right now we have a huge list of exclusion tags to work around serious issues with Tidy. Which isn't Tidy's fault. It was made before things like SVG and the spec it follows it follows properly but epubs need a lot more than what Tidy offers. Hence why it's being replaced with something modern that not only respects html5 but also (hopefully) will respect epub specific tags and tag prefixes.

Thanks for all your work on this!

FYI, tidy is no longer part of Sigil master at all.