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?
Plumbing – What tool is used for this tight plumbing situation
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Related Solutions
Similar tools are also available for automotive trim removal. They are often called "automotive trim removal tool" (go figure, eh...)
It may be time to call in a professional with a piping inspection camera system or rent such unit yourself and do an inspection of the drain line from the tub down into the sewer. This would clear up any concern that there may be a blockage or loose article that has entered the drain system.
The report that you had already removed a bunch of acorns from the roof vent line leads to concern that this vent is still clogged to the extent that free air movement below the tub trap is not being allowed and that the tub drain ends up with an air lock and thus cannot drain properly. It may be well to also scope out this line down from the roof with the piping inspection camera. (Do also make sure to install a screen on the vent pipe to prevent insertion of more acorns).
Since you already made attempts to clear the roof vent line is it somewhat possible that the vent was never installed correctly from the original construction and that this tub never drained properly. If that is the case then it may become necessary to install an additional vent pipe. The need for this of course could be eliminated if it was known of this particular bath tub did drain properly in the past. That could come from your own experience or consulting with previous owners.
Best Answer
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.