terlouw,
Your #4 is a little orthogonal to the design concept of RoundCube. The RoundCube architecture is centered around a highly interactive JavaScript front end interacting via AJAX with a PHP back-end that talks to an IMAP server and SQL database. That architecture doesn't seem to lend itself very well to mobile devices, due to both the limited JavaScript support and the frequent asynchronous server interactions.
A more static-HTML based web client (or an actual mobile IMAP client) might be better suited to mobile devices.
-Eric