My cover pages (back and front) were done in a Mac program called Swift Publisher I like to work on.
1) Should I can download Inscape?
2) I'm thinking of doing this any way and will it require Quartz?
https://inkscape.org/en/download/mac-os/
Here's a vid of how it works for the conversion process:
https://www.youtube.com/watch?v=1cZk08x_rAI
Or SP will export as: PDF, TIFF, JPEG, Portable Network Graphics image and EPS
3) Are any of these usable for my project in Sigil/Calibre?