The picture of the K-200 is, I'm certain, largely speculation and post-war speculation at that. I asked Shorzoe Abe about the K-200 and he had never heard of it and this coming from a fellow who is pretty familiar with Japanese aircraft, especially Kawanishi.
The position of the engines is to protect them from ingesting sea spray which, I'm sure, would quickly corrode and destroy engines already fickle.
Kawanishi may have designed the K-200 but I suspect they didn't put much stock in it. First of all, they had to wait until the successors to the Ne 20 were produced as the Ne 20, even six of them, would not have granted the K-200 any respectable speed, let alone heave the boat off the water. Second, turbojets are very thirsty and the bulk of the K-200 would have been fuel to feed all of those engines. By consequence, the K-200 would have had a pretty weak operational range that was less than flying boats already in service, namely the H8K. In addition, reliability of those early turbojets would have been suspect. With operating lives in the few hours, the K-200 couldn't wander far from base lest engine failure strand it. Unless the K-200 was to be built out of wood (like the H11K), there was little metals to spare for such a project. Finally, Kawanishi was already swamped with other work, namely fighter production after having been told to concentrate on them in lieu of flying boats. This issue saw the K-60 canned. The loss of the mock-up of the H11K Soku also saw no further interest in the big flying boat in light of the late war situation.
Interestingly, that post-war speculative illustration of the K-200 has a resemblance to the Beriev R-1.