I used to run this version of enlightenment + slackware(?) on a 25MHz 386 with 8MB (or was it 4MB?) of RAM:

http://www.linuxfocus.org/English/July1998/article52.html

Compared to windows 3.1, the default theme flew, and visually, it was way ahead of Win 98. I remember getting pseudo transparency working, but I think that was in the pentium 100Mhz days. Other than that, it checked all the other boxes on your list.