Ice accretion is a very complex phenomenon on airplanes that depends on several factors. One of the main factor is the composition of the cloud that the airplane is flying and the exposure time.
Another significant effect are the conditions on the imprigment surface on the airplane, which depends of the flow path and the pressure over the surface.
So, having the properlers rotating in the same direction means that the propellers are providing energy in a different direction. Also that will mean that, for optimizing the airplane, the fuselage will be slightly assymetric (very difficult to notice).
So, to the first statement you are mentioning, I can confirm you that the imprigment areas affected by this assymetric effect will develop different ice shapes.
What I can tell you is that FAA and EASA employ real experts for airplane certification and they are concious of this effect, and I can tell you that any doubt they have about aircraft performances will request the manufacturer to demonstrate in flight.
My answer to your question is that those airplanes are not protected in the sense that they will develop assymetric iced conditions but are strongly protected in the sense that this condition is considered and very likely tested with airtificial iced shapes, so the airplanes are really safe. So globally, yes, the airplane is "protected" by design.
For your questions about 4-engined airplane, the answer is the same but having more complexity due to the number of engines.
I can tell you as well that in iced conditions, due to the complexity of the phenomena and the prediction and simulation difficulties, there is a lot of conservatism in the way manufacturers design the airplane and significant cost in airplane testing. FAA and EASA are really doing a good job.