Running independant return from each zone does mean more pipework yes.
When you say TRV's to the left that makes no sense, TRV's should be positioned accordingly to the flow of air in the room, for example a TRV on the left of a rad and a window on the right of the rad means the section of room with the TRV could be slightly hotter due to drafts from window, putting a TRV on the right in this case should mean equal teperatures.
Reverse circulation will occur when water is pumped through the coil of the cylinder, the water still carries enough charge from the pump and works it way though the returns of the heating circuit/circuits. If you were to tee the returns in in the cylinder cupboard and use non return valves it would stop reverse circulation, however should one of those non eturn valves ever fail then you would get reverse ciculation, this will cause boiler inefficiency (how much depends on the system/systems) and you probably wont know until your next fuel bill. If you run independant returns you should never get reverse circulation and in turn boiler inefficiency, even if the water returning still carried some pump charge the pump will then be sucking that water through the boiler and pumping back round the circuit/circuits.
P.S unvented hot water cylinders makes no difference to potential reverse circulation.