All questions are really on spot.
No, then only thing you may do in this case is not placing ground plane under the MagJack. I researched the matters and decided that for hobbyist projects this measurement is too much. Professional projects use external magnetics anyway, which can have grounds properly separated.
Do not partition digital and analog grounds (even with ferrite bead), but partition 3V3D and 3V3A with ferrite bead - this should be enough. In any case you must ensure your power source is clean at digital side.
I have taken arbitrary value of 30. Probably took this value from Arduino Eth Shield design 
