Piggybacking a ECU package

Hi there, I am considering going down the route of piggybacking my Autronic SM4 ecu via a patch harness into my JZS161 Aristo (automatic). I am a bit confused as to how go about the wiring;

1) How do I go about tapping the crank, cam sync, water temp signals. Will there not be an issue if both the stock ecu and the Autronic tap into the same signal?

2) Do I let the stock ecu control the idle control valve?

3) What do I do about the sensor ground signals (injectors, cam, crank etc.), Do I need a separate ground line or do I tap into the same stock ECU's ground wires.

4) Similarly for the +12v and main Ground signal can I tap into the stock ECU's source?

Hello on the autronic it will be a nightmare i have tried to do similar and gave up after finding the Link kurofune with a comprehensive help file it is designed to be used as a piggy back ecu

Regards Ross