We have a small yard that is green. Some of it is grass, some crabrass,
some weeds & some clover.

I like the grass and the clover, but I don't like the other stuff. Will
a pre-emergent kill the clover & all the other stuff?

Do pre-emergents have runoff problems or wildlife consequences?

Is that all I should do in spring?

Should we overseed now?

I really wouldn't mind if the lawn was all clover; it's the really short
kind with little white flowers, it attracts bumblebees, doesn't need
watering, fixes nitrogen and doesn't need mowing. What are the odds of
it taking over?

TIA for your patience.