The idea of more than four engines was actually fairly common, particularly in the early days of aviation, due to the relatively low power output from most available engines. The problem isn't an inability to think of the idea, or even an inability to build the idea, it's 'diminishing returns'. Adding an extra engine adds (obviously) the weight of an extra engine, and the weight of extra fuel, and also the weight of extra structure to make room for the engine. That makes a larger aircraft, and that increases drag, which along with the weight, increases fuel consumption. Eventually, you reach the point where that extra engine you added is basically lifting what it added to the aircraft...and the next one you add won't even be breaking even. As engines increased in power and reliability, designers could meet their power requirements with fewer of them, and that let them benefit from the opposite side of the above problems...airframes could be smaller, with lower weight and lower drag.
As for saving the Kalinin K-7, it would've been possible to do so, but why bother? The K-7 first flew in August of 1933. The B-17 first flew in July of 1935. In other words, by the time the bugs would've been ironed out of the K-7, there were aircraft entering service that flew twice as high, carried more payload, were more mechanically reliable, and were much faster, to boot. It's a cool looking aircraft (in a clunky sort of way), but it wasn't a very practical one.