Motorised valve position help needed.

If you put a zone valve on radiator circuit then the hot water circuit could be working and extra risk of pumping over on some systems with vent and feed ahead of pump.
Sealed system best, if suitable.
Good that you are doing the wiring to use the zone valves to turn oil boiler and pump off

Whilst I understand financial constraints I believe the temporary fixes tend to end up being permanant and don't end up getting done properly.

Another thing to check would be get a cup of water and with the system running put the cup up to the open vent pipe to see if your getting air sucked in through OV pipe (the water will suck up from the cup if you are)

Sounds like you intend for the valves to be wired in properly with boiler interlock so that's good as otherwise I'd imagine you'd definitely experience pumping over when zone valves shut off.

Personally I'd take the time and little extra effort to alter the pipework slightly and fit 2 zone valves (wireless cylinder stats and roomstats make job fairly easy with wiring) for long term performance and ease of maintenance of the system.

I'd also check water quality when fitting the valve(s) and inhibitor levels.

Ok, update for anyone interested.
Fitted second MV for central heating and auto bypass and all working with no problems so far.
Will keep an eye out for pumping over but none at present with either hw only or ch only or both on/off.
Also went for a room stat that I can control via my phone and it's great. Can set as many times and temperatures as you like so that will definitely pay for itself.

Well done rodders you knew it made sense