Mare of Earth
1) At the moment someone has managed to use 16 000 books. (with disabled card scanning). There is hope to solve this problem using mount/umount in the future versions of PRS+, then there will be no (reasonable) limit.
2) Hard. It requires not only good programming skills, but also outstanding reverse engineering skills. Igorsk
has found a way to write custom viewers, but... It didn't work for me (probably while API has changed a bit meanwhile).
Last, but not least, having calibre and many other tools that can convert between formats in a mere seconds, is an in-ebook support really needed?