What do spawning bass normaly do when the lake is coming down and they are already commited to a bed.

There in they can do since they can not control the down draw. They certainly can not pickup their eggs & move.

If the water drops to the point the bass will not survive...abandon nest, hopefully the female spawns again.

That's why water levels are very important in the spring.

The female bass will in fact spawn with several males on different nest thus insuring the population.