The two fixes for the HTML problems, respecting the system font size and the preference to turn off full html, have been released. See
Calibre Companion V3.1.1 released
@PatNY: I accidentally discovered that the Remove Formatting button in calibre's comments editor removes font and size changes. Select the entire comment and tap the button. It leaves all the paragraph, header, and div tags, but it removes centering. It is probably easier to add the centering back than to remove the font changes one by one.