^Single cores allow hardware multithreading since forever. Each Xenon core runs 2 threads. And there's a great difference when coding for 1, for 2 or for 6 threads.
Pushing the GHz count is simply not optimal for game development, where you're facing highly parallelizable tasks. That's why both MS and Sony moved to many-threaded, in-order processing. I naturally assumed that if Nintendo went for a higher profile CPU they'd do the same.
A single PPC out-of-order CPU even at 2.4GHz would be way weaker than either the 360 or PS3 in every real gaming case. And talking of the GPU frequency without speaking of its architecture when it comes to memory access or shaders makes even less sense.
Please, let's just cut the technical part.
As to your last paragraph, you're basically not answering. You're assuming that a Wii with higher hardware costs, a different market of third party offerings and a higher per-game final cost would ultimately just somehow end up in the same profit for Nintendo. That's a lot of magic right there :)
And the higher software production costs would not be exclusive to first party titles, you know. They would also change how third parties approach the platform and ultimately the license revenue.