I'm curious, is your dash toggle switch to manually operate the fan all that necessary? You say you need to pay attention to the coolant temperature using the switch. If you were to just leave the switch on at all times so it's always operating, how different would things be watching the temp needle on the dash? As long as your dash bypass switch is hooked up using the old leads on the rad thermal switch, the fan relay will only be energized when the ignition switch is ON (KP-II).

My guess is it will take slightly longer to reach normal operating temp, but otherwise you can just leave it on and let the cooling system thermostat do its job looking after maintaining normal operating temp. This wouldn't be much different than having a full time belt driven fan (as the 1989 240s had) other than the fan would run in KP-II whether or not the engine was turning over, so could run the battery down. Also, in the later 940s without the thermal rad switch, the fan was on full time whenever the A/C was turned on and running (using a pressostat on the condensor as the A/C sensing switch) in addition the ECU being able to operate the fan on demand whenever the block temp sensor said the engine was approaching hot. Electric fans were primarily introduced to slightly improve overall gas mileage, slightly lessening the load on the engine, and the rad switch was there to cycle the fan, lessening the demand on the battery and alternator as well as reaching normal operating temp sooner for slightly improved gas mileage.
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Dave -still with 940's, prev 740/240/140/120 You'd think I'd have learned by now