Hey there.
Someone will stop by soon and answer your questions about distribution, but at least I can answer about the tech stuff

The main menu is customizable, but not using the Studio. When you submit the MOM, we'll be able to edit the texts to be more relevant to an Ebook. If you have any suggestions for the texts, feel free to send them - it would help

Users will be able to alter the main menu more in a future version of the Studio.
If you want to translate the texts to another language, it will require either a different MOM file for each language or to have the language be selectable inside the MOM (using some variable to keep track of which language to display). As long as you use the same font and font size for each language, the second option should work fine - otherwise I'd recommend different MOMs.
Regarding the font in the main menu, it's possible to use any font type in your computer - starting from the next Studio version, the font will be bundled with the MOM so that we will be able to use it when compiling even if it's a nonstandard font. However, it's not possible to use images for the texts in the main menu. If you'd like to use a font type you created, you can turn it into a True Type font and import it into the Studio (starting from the next Studio version).
Hope it helps.