will the following ground plane layout be valid for noise immunity/suppression?
The quiet ground is essentially a plane for a few components as per the recommendation in the A3941 data sheet.
List the currents in your design, in descending di/dt order. On top of the list you'll probably find the input and output currents of your motor driver. Then, keep these currents out of the parts of the ground plane where sensitive circuits are. So... this is all about placement.
I don't see any connectors on your drawing, and that's important. Since there is a power supply section, I suppose the power entry connector is there. And I don't know where the connector for the motors is, so I put it on the edge of the board closest to the drivers.
Current will flow from the power entry connector, through the board, in the ground plane, probably a bit like I drew. So it is going to nuke the potential of "0V" everywhere. Also, if the motor connectors are on the other side of the board relative to the power entry connectors, and your ground plane has high impedance because it's full of cuts, then there will be a potential difference between GND on both connectors, which means the cable will radiate common mode noise, and it will fail EMI.
So, it's simple : put the motor drivers right next to the power entry connector, with some decoupling caps on the DC rail in between. Then the motor connectors as close as possible to the power entry connector. All on an uninterrupted ground plane, and on the same edge of the board.
Then the high current won't go through your whole board, it will stay in that zone and follow the path of least impedance.
Basically, keep the area of high di/dt loops small and compact. The smallest area is achieved when forward current is on a fat trace or pour on layer1, and return current is on the ground plane on layer 2 right below.
If you use a differential current sense amp, remember they have pretty good common mode rejection from the sense resistor to the differential input, but the output is referenced to whatever "GND" is where you connect the "GND" pin of the chip, so you can make the differential pair longer to place that chip away from the noise, or pickup the reference from a quiet spot.
And, since you should never route traces over ground splits, when you get rid of the ground plane cuts, you'll find your routing is a lot simpler and you have plenty of space left. Also if you shorten the high current paths, you'll have much less fat traces taking space. So you'll have room to push the switching converters in a corner away from the sensitive bits, and just isolate with a bit of distance. I suggest to put the switchers on the other side of the power entry connector from the drivers, so the switchers don't emit noise into the motor cables.