All good questions. I have found that using a center tapped ground is preferable in the case of high current power amps, but in something this small, it's hard to say... the only advantage I could see if the layout was simplified by using this connection, and the loop between the transformer and first smoothing cap kept as short as possible...
TVS are typically more precise than a MOV and they operate at different speeds. I remember reading about a combo gas bulb / MOV to get the best of both worlds there, but in this case paralleling made sense because of the much larger voltage that the gas bulb could handle... I'd suspect that one or the other would do just fine. Also, the capacitance of each might be of concern, I didn't check that.