1) the Sansa e250 is supported by Rockbox, but only the v1 version of this player, not the v2 (it has complety different hardware inside). You can best check this with the Firmware version of the original Software on your player. If its v1.xx.xx.xx rockbox works, if it v3.x.x.xx its a e200v2 and rockbox doesnt work.
2) Yes rockbox works fine on a ipod video (not classic !)
3) Rockbox offers both ways of navigation, a normal filetree and a database. you can choose :-)
4) In Rockbox you can change what is displayed while playing music (called WPS screen). There are many User created Themes for it, with and without Albumart. You can even create your own.
5) please take a look at a rockbox manual :-)
greets,