I think you are looking for a magic bullet and I don't think one exists.
The fact that your device balks at doing typical overrides, leads me to believe that something 'tricky' was done or you have a
fixed layout book.
I gave up trying to use CONVERSION to fix layout issues a long time ago and use the editor almost exclusively. It can be lots faster in many cases and does not change anything that you did not do yourself

.
HTML and CSS (as used in ebooks) is fairly easy to understand. IMHO understand the 'Box Model' first, because everything on a page is really boxes within boxes (A paragraph is a box. A Span is a box within another box. Some parts of the box may not be active, but they are still there: like border:0, padding: 0.)