1. Sure it is easy to do, open a bug for it
2. It does not do anythong to compress, it simply has a very efficiently designed PDF backend that I wrote from scratch.
3. All calibre conversion settings are saved.
https://manual.calibre-ebook.com/conversion.html (see the section on how settings are saved)
4. THere is no way to gruarantee "correct" conversion of a dropcaps, just as there is no way to guarantee correct display of one. It depends on the font size/dpi/other css settings, you will need to play with them.
5. The best way to display images with margins zero is to use the standard svg cover markup that calibre generates for its epub cover pages. That is guaranteed to work.