Our tub drain is severely plugged. Cannot get a snake down into it. Our crawlspace is only about 18" tall (WHY?!?), and due to both the confined space and the angle we cannot leverage the wrench in position to loosen the clean-out plug. In addition, the clean-out plug is VERY tight. What is the ideal tool for flexibility, strength and leverage in this confined situation?
If you haven't already, the cross-hatched drain cover in the tub can be removed with this wrench, called a bath tub drain wrench (or similar):
There should only be plumbers putty underneath it, so it will be hard to turn at first, but then quickly becomes quite easy.
Once that is out of the way, running a small drain snake down the drain is comparatively easy. There is probably no need for you to go under the floor.