Encoder PCB design - grounding, noise and decoupling
To answer your first question, differential signals do not have a return path.  There is no return through ground.  That is a property of single ended signals.  For this reason, differential signals don't theoretically even require a common ground between the two end points, though don't do that in practice unless you know what you're doing and using isolation of some sort (like Ethernet does).  The whole point of a differential signal is it creates its own return.  It returns through the negative line, and current flows from the positive line to negative line and never flows into ground.

As for your layout, it's ok, but you absolutely need ceramic decoupling capacitors.  Electrolytics have a lot of ESL and are effectively resistors, not capacitors, for frequencies above 100-150kHz.  So they will do nothing at all about ringing.  Because of that, you are only decoupling the frequencies below that from the rest of your board.  You must use ceramic bypass capacitors on the input and output if you want to attenuate the higher frequency ringing from being felt by the rest of the circuit.