your favas, yes, will behave like beans and make your soil fertile.
Their taproot is well adapted to clay soils. But you should have
planted them in september for an april crop. My favas suffer when
summer comes, they pick up all sorts of diseases.
People have mixed experiences about those powders (I do not use them,
but I have some composted materials in there which contain similar
bacteria). I don't know what you mean by a decent crop. Since they
freeze beatifully, for me a decent crop is 50 lbs. 6 fava plants will
probably give you enough to eat once. Picked young, they taste very
good.