Convert to KF8 instead of Mobi.
Use Calibre to edit the KF8. Then add the following to the top of the CSS. You will need to figure out the correct negative px to get a 0 margin. You will need larger then -45px. But once you've figured it out, you'll be good to go.
Code:
html {
margin-right: -45px;
margin-left: -45px;
break-inside: avoid !important;
}