I assume that you have modified your mplayer.conf. That goes a bit too far to parse that file I guess.
IIRC, there is a way to get the system volume as a control. The problem is where to put it. The interface, due to the playlist is already quite full.
If you use mplayer with omapfb then the buttons don't work because mplayer isn't attached to a X11 window.
Hmm, the enter key should indeed pause and unpause, I check that out. The up/down keys does position the playlist in fullscreen. Only for mplayer w/o omapfb this is disabled because of mplayer's own hardware keys.
There is a position slider. It works as long as the stream has a length set and updates its position.