I'm not at all upset Dick.
It just appears to me that you have taken the question and attempted to justify the examiner's answer. I have looked at it, and considered what the correct answer is.
But let's look at your idea of a powerful engine and constant speed prop giving a flat THP curve at Vmp.
Most texts state that for a basic low powered aircraft with a fixed pitch prop, Vx is just less than Vmp and Vy is just more than Vmp.
For Vx we need maximum excess thrust. This means that as speed increases towards Vmp, the benefit of decreasing drag must outweigh the cost of decreasing thrust, such that excess thrust is maximum just below Vmp.
For Vy we need maximum excess power. So for Vy to be just greater than Vmp, this means that as speed increases to just beyond Vmp, the benefit of increasing THP available must outweigh the cost of increasing power required.
The important points here are that thrust is decreasing and THP available is increasing as we approach and pass Vmp.
To create a flat THP available curve at Vmp we would need to have a situation in which the thrust reduces at exactly the right rate to balance the increasing TAS, such that THP = Thrust X TAS = Constant.
Well, if we already have increasing THP at Vmp with a fixed pitch prop, why would we design a constant speed prop that causes the THP to be constant at Vmp?
I agree that a flat THP curve at Vmp would give Vx = Vmp. But I just cannot see why we would want to create a flat THP curve at Vmp, because to do so we would need to ensure that as speed increases, the thrust decreases even faster than it does with a fixed pitch prop.
I know that this type of question has been appealled in the past. If you cast your mind back to who was managing PERF for the CAA at that time, you will not be at all surprised that the "improved" question is still a mess.