Those numbers were chosen by Ralph, I have no idea why those were specifically chosen over a random range.
Also, those genes for Emily were chosen by Ralph, it was something about being optimized for breeding, though that may no longer apply unless the equalizer setting is changed.
There are a couple solutions to your Demon breeding problem. Edit the save file to give the player something like 80% demon and 20% human; it won't update the genetic bonuses but you'll get the children you wanted. Change "player_secondaryrace" and "player_secondaryracepercent" in expansionsettings.gd and start a new progress, though I'm not certain that it works well in every case.
The fetish system is still rather new, so I expect that it will change a bit in future versions and the other issues will eventually get addressed. If you wanted to make sure the start slave quest ends as expected, then you could influence them to have the fetish opinion high enough to get that trait.
Edit: The save file is JSON formatted, so I recommend using an online JSON editor for editing the save file. Also make a backup if you do, it's hard to fix save files.