a) root is read-write, and software can be installed there, as in normal Linux. However, because space is limited, even though root is compressed, they install in the 2G partition, mounted in /opt. The process of moving off root and in the 2G partition is called "optification/optifying"

b) You can't back up the radio driver, correct. Why older versions don't downgrade the driver I don't know.

c) There's also the card, bringing the grand total to 50G ish, it's mounted in /media/mmc. 16 G max because that was the largest this far, but people report no problems with 32G microSD. Why would anyone use more I don't now. But, why not?

d) Also, there's a partition you can't see, for swap (754M).

e) Most of this info is in the wiki. It's a good read at your own pace, includes OC, custom kernels without OC, well, everything you want to know about N900. (Wiki.maemo.org)