Tell that to your mail program. If it chooses to show you the mail that way, don't blame me. Everything needed to display it any way you want is there, it just needs to be used. Thunderbird can define a minimum text size and refuse messages to use their own font. If your archaic software doesn't do basics, blame the dev - or better yet yourself, as the choice is yours.
And the other part is its own thread. I've commented everything I know. Just redo your book as native ePub, there's no way around it. If you want to find shortcuts, you'll have to do your own research, even beyond Linux there probably is no piece of software that can do what you are looking for. But on the other hand, I'd never have expected ghostscript - or to be more precise GhostPDL, if I'm not mistaken - to be able to handle Microsoft's rubbish XPS format and convert that to a proper PDF. So who knows? Instead of going on other people's nerves with an unsolvable issue, put those questions into the search machine of your choice. Maybe it will be more competent than your mail program.