What is the shell of the DSUB25 serial connector on the MT2834ZDXb electrically connected to? Is the signal reference (Pin 7) of the serial port common to the logical ground plane of the modem? Is pin 7 isolated from the wall power supply?

The shell of the DB25 does not connect to any ground except when attached to a PC, then it terminates to the PC's chassis.

Pin 7 connects to logic ground.

The AC side of the power supply is isolated from the logic ground of the modem.