You are way overthinking this problem.
If you use the grounding path the derale controller gives to command fan on and just splice it to the ground point for the stock GM cooling system RELAY ground path, it will function properly.
The same wire that goes to the ECM from the ground side of the GM RELAY itself is where you want the derale ground to go. It wont matter if the ECM grounds the relay at 234 or the derale controller grounds it at whateverlower temp you set it too. As long as One or the OTHER grounds that circuit, the fan will run. This also eliminates the human error involved with flipping a switch if one system fails. If the Derale system fails to ground the wire to the fan relay, the GM system will act like a redundant or fail safe to turn the fan on. Simple.
Fact is stranger than Fiction