Hence, I still do not understand the logic why isolating only a subset of walls instead of all of them would be a problem.
As I said, I had an energy efficiency specialist at my home which said that isolating 3 walls would be 75% as efficient as isolating 4 and totally recommended. So far, I have not seen any solid logic supporting the fact that isolating only 3 walls instead of 4 would *increase* the risk of mold compared to either, 4-walls all insulated, or, zero walls insulated.
As the house cools at the end of the day the uninsulated wall will be colder than the other walls and if it's below the dew point condensation will form on it. If the temperature differential is high enough in comparison to difference in insulation between the walls then enough water will leave the air to keep the dew point below the temperature of the insulated walls and so that one wall will collect the majority of moisture (at 30C a meter cubed of saturated air carries about 30mls of water so yes condensation on a surface can heavily impact humidity). Once a wall becomes moist it will still acumulate water if it's below the dew point.
You likely will have seen real life examples of this when you look at single pane windows in otherwise insulated houses and seen them fog up/ have condensate form on them while the walls around them remain dry (if you haven't seen this but have been to houses with single pane glass in your area then condensation won't be an issue where you live).
As to your 75% efficiency point it will in fact have a lower efficiency as heat transfer increases with temperature differential. Since the room has a higher temperature differential with the outside the less insulated wall will be faster at transfering heat.
That is not to say that insulating 3 walls is a bad idea, it will almost certainly improve a rooms U value, but the worse the insulation on the remaining original wall, ceiling, and floor the less difference the 3 walls will make to the rooms U value and thus how quickly the room transfers heat.
