1.. The modem speed in Putty, is not used because the connection is made to the USB port, only when you use Putty to connect in old fashion using a DB9 connector, so called Serial port. You can forget the speed.
Further to the speed this is of no importance as the connection is only a control connection to send the few charaters in the AT string, this speed has nothing to do with your InterNet connected speed. Putty is only use like the ignition key in a car, to start the car, shutdown the engine, lock doors etc but has no influence on what the car does in the mean time. A value of 38400 would be safe but if it connects with the higher value you have don't worry about the speed figure at all.
2.. I don't think it is the eth0 causing you any problem with the connect files. The eth0 is only for local area network, your cable connection to a modem, which you don't have. The Sierra 320U connects via USB to computer and Wireless method to Bigpond radio towers in you neighbourhood.
3.. The problem you have I think could be another, from root konsole post:
Looks like we need to look at using dhclient -r
and dhcclient -nw usb0
I don't have time at the moment. You could join up with Whirlpool and discuss the problem with the tough guys there.