Yes you could use the WAG354G as a wirless access point between wired and wireless networks, You could even connect up to 4 PCs together wired.
What I can't see is how you would get it to work as a router on cable, since there router function is wired to the ADSL modem, and you can't bypass that? For the device to operate as a router, and thus allow the internet connection to be shared a modem has to be connected on the WAN side of the device and the computers on the LAN side. The only WAN connection is the ADSL port, expecting to wire into the telephone network, not cable.
NB. WAN = Wide area network - the connection to the internet side, LAN = Local area network- the computers you have connected up in your house.
As for the USB to ethernet adaptor, those usually work by converting ethernet to USB rather than USB to ethernet, and thus probably wont suceed in passing your cable modem's signal from the modem to the router's WAN port.