Overheating: Yes, probably.

Terrible Idle: Maybe, could be temperature or flow related.

White smoke: No, probably not, unless that passage being blocked has pushed coolant into the combustion chamber somehow. White smoke FROM the exhaust (Make sure it's actually puffing the tailpipe) isn't terribly unusual for most cars on COLD cold startups. If you're talking about cold startups as in ambient temperature is around like 70-75 and it's the first run after four hours or so...then you have coolant leaking into the combustion chamber when it's cold, which most likely means a headgasket, as that's the easiest path for coolant to combustion. The gasket might not be shot, just not 100% sealing when the engine is cold.


Second thought: Engine may have overheated around the blocked coolant passage, causing a warped head/block. Your gasket may look fine but only seals when everything has expanded and warmed up enough to seal fine. Leak is probably very small. Blocked passage would DEFINITELY cause the area that's no longer getting coolant to superheat, and this would likely be the cause of the warping.

If the head/block is warped just slightly, then yes, that one blocked coolant passage is indeed the whole reason why your car is running terrible. The bad idle could be related to low compression. Does the idle get better as the car warms up?