I think that for lakes A, B, and C, all the options you suggest look absolutely fine. With the big middle lake, the outlet 6 looks fine too, and I don't see why there shouldn't be that smaller lake on the way. If you want to keep river 17, why not just extend it so it flows into the big middle lake?

The question of how many rivers to show is really just aesthetic. If this were a real landscape, it would have rivers of all sizes, right down to tiny little streams that you couldn't possibly show on a map of this scale. Someone making a map of a real landscape has to decide how big a river has to be before they show it. So if you think the map looks too empty, you can add little tributaries for the rivers. If it looks too cluttered, take some of them out on the understanding that they still exist but aren't shown because they're too small.