Hi, yes, I disconnected the Ethernet cable from the Bridge II and left it that way for many weeks before physically removing the Bridge II from the DS. As I recall, disconnecting the Ethernet cable resulted in a sonic improvement, but in hindsight it was minor compared with the big change that resulted from removing the Bridge II.
I never tried optical isolation of the Ethernet input. Maybe that has a similar affect to removing the Bridge II.
As for I2S, I have been using a Matrix X-SPDIF 2 for over a year to convert USB to I2S and I am happy with the results. There is a thread about the Matrix here.