re: Re-using the engines
They wanted to re-use as much as possible....The engines being quite complex and expensive, wasn't it logical to want them re-usable too?
The External Tank is a tank with a few tubes and hardly anything complex.
Of course, the entire stack turned out to be more difficult to operate and that's why , perhaps, the engines lost their re-usability appeal, but it wasn't planned that way in the 70's.