Shunt resistors and PCB layout
If your sense circuit is well designed (high input impedance say > 1K- and good common mode rejection) it should be quite insensitive to resistance in the sense lines, so matching is not required and the resistance of the vias and traces is of little importance. Of course it would be better in general to use parts with four connections but your method probably is pretty good.  

Cutting the ground plane may have other effects, especially if high current traces cross the cut, but sense lines should not be affected. 

Trace width is of little importance if your sense circuit is well designed, but you may wish to keep them close together (specifically minimize the loop area of the differential input) to minimize susceptibility. Usually the circuit is quite low impedance due to the shunt resistor source so it's not really that critical.