the venice requires 50R impedence. IN all honesty, the missmatch between 75R and 50R isn't going to make that much difference.
I've frigged up a passive multiplex unit but you end up losing some signal. An active multiplexer would be better but in its current state it will not run from the USB any more. I have made a smaller more power efficient unit but I am unsure if Craig will sell this or not.
I have used both the venice 5, 5.1 and 7. There is not much to choose between the three. Through london i had no problems with the 5 or 7. The serial control is better on the 5.