The photos in this post are very similar to my own setup. I've used this photo to get an idea of how the standing waste overflow works.
I'm as stuck as my tub on this issue. I tried Liquid Plumr early on and that kind of worked and got some water to go down very, very slowly after some hours at one point but that didn't seem to fix anything as the next time I filled the tub with water, it wasn't draining. Which, this is my problem; the tub is just clogged and not draining any water.
After trying liquid Plumbr, I tried vinegar and baking soda along with hot water and that didn't work. Next I ordered a hand cranked drill auger and an electric drill auger and I wasn't able to have any success with either. When I use the drill augers, where I'm inserting them into the standing waste pipe, they go straight down, and then that's it, it's like I hit a wall. I can't seem to get them to do down where I assume is the low seal trap. I've tried poking down a metal coat hanger that I straightened to feel my way around down the bottom of the standing waste pipe to get a better sense of the situation but again, it's like I'm hitting a wall and I'm at a loss as to what I should do next.
Best Answer
I know you don't want to hear this, but as a plumber/maintenance person, I will tell you this - There is no science to your dilemna. When we are called to your issue, we will try the same things you did, only we never use chemicals before snaking. I only use chemicals for under-concrete-pad-or-crawlspace hopeless clogs, because I use acids only available to licensed plumbers, that will melt your pipes if left to set too long.
Other than that, it's persistance. Keep jamming coat hangers, augers, electrician fish tape, whatever you can dig out of your truck or closet, into that booger until you pull out a plum. Without getting underneath the plumbing and physically seeing your piping, there's no way to know what you could be hitting.
There is a water jet drain opener, but that could get messy fast if you've never used one. Google "water jet drain cleaning", maybe Youtube it as well. The tool is about $10 at Walmart, and only needs a garden hose.