will the Ethernet shield work stand alone for this task if supplied with 3.3V?
The Ethernet Shield (the one from Arduino) might do this job although I never tried it to use this way. The "shield" (is a breakout and no shield) you had a picture of in your first post looks like an ENC28j60 and that may react completely different.
I'm looking for a really "unmanaged" solution that is just plug an play.
I'd probably use the USB-powered Gb switch for you task. It offers speed recognition and it detects the link far more reliably than the discussed solutions, and it's probably much cheaper too.