Regarding reference planes

If we are thinking of a single ended conductor it's easy. Imagine the conductor in free space & imagine the field magnetic and electrostatic around the conductor. Now imagine where your planes intersect the field above and below. Both planes intersect, so both will have induced currents and both will modify the "shape" of the field.  The sum of the induced currents will be equal and opposite to the current in the conductor. These currents form a loop, out of the TX pin, along the conductor, into the RX pin, through the RX chip and into both referenced planes,  back along the planes  and into the TX chip power pins, finally going back to the TX pin.

The return currents in the two planes will try to follow the path of the single ended conductor, because if they don't a loop is formed. Look up the equation for inductance. The larger the loop area, the bigger the inductance. So if a loop is formed, a magnetic field is formed and work must be done, energy is expended in the form of radiation. Electrons are basically lazy they won't work unless we force them to, the the return currents in both planes follow the single ended conductor as closely as they can.

When you jump a plane spit with a high speed signal, you're forcing a loop. 

Now consider a diff pair. The field strength will be greatest between the coupled edged of the diff pair conductors, but the fields don't exist in this space only. Each conductor has it's own field surrounding it. That field will induce a current in any conductor it encounters. So yes, return currents flow in both planes here too.

Engineers seem to get hung up with the idea that the current in the +ve and -ve halves of the diff pair balance each other, and that one forms the return current for the other. Well even if you have a perfectly current balanced differential transmitter and you have no skew between the pairs and even if you have no bends in your diff pair (so the signal in one half can't overtake the other) you will still have currents in the planes.

I recommend a read of Dr. Howard Johnston's Black Magic books.

If we are thinking of a single ended conductor it's easy. Imagine the conductor in free space & imagine the field magnetic and electrostatic around the conductor. Now imagine where your planes intersect the field above and below. Both planes intersect, so both will have induced currents and both will modify the "shape" of the field.  The sum of the induced currents will be equal and opposite to the current in the conductor. These currents form a loop, out of the TX pin, along the conductor, into the RX pin, through the RX chip and into both referenced planes,  back along the planes  and into the TX chip power pins, finally going back to the TX pin.

The return currents in the two planes will try to follow the path of the single ended conductor, because if they don't a loop is formed. Look up the equation for inductance. The larger the loop area, the bigger the inductance. So if a loop is formed, a magnetic field is formed and work must be done, energy is expended in the form of radiation. Electrons are basically lazy they won't work unless we force them to, the the return currents in both planes follow the single ended conductor as closely as they can.

When you jump a plane spit with a high speed signal, you're forcing a loop. 

Now consider a diff pair. The field strength will be greatest between the coupled edged of the diff pair conductors, but the fields don't exist in this space only. Each conductor has it's own field surrounding it. That field will induce a current in any conductor it encounters. So yes, return currents flow in both planes here too.

Engineers seem to get hung up with the idea that the current in the +ve and -ve halves of the diff pair balance each other, and that one forms the return current for the other. Well even if you have a perfectly current balanced differential transmitter and you have no skew between the pairs and even if you have no bends in your diff pair (so the signal in one half can't overtake the other) you will still have currents in the planes.

I recommend a read of Dr. Howard Johnston's Black Magic books.