svg cover images are always centered and can preserve aspect ratio or not as per your preferences. And
when conversion inserts a cover it
always uses SVG,
unless you set the option to not use SVG.
As for the rest of your questions, trying to guess what you are actually doing from your descriptions of what you think you are doing is too difficult, see
https://www.mobileread.com/forums/sh...d.php?t=186697