Please help! It's always been a slow slow drain, but this is crazy. We tried plunging (smelly black gunk came out), we tried drano, we tried the stinky enzyme bacteria stuff...

Today in desperation I bought this compressed air thing, and it seemed to work, for about a minute. I had standing water, covered the vent, and pressed the canned air in. I got sprayed, and then lots of bubbles came out, and then it drained quickly, and I thought it was fixed. I went to wash my arms and face from the yucky water, and then it stopped draining again. You can see and hear that the water only goes down a little at a time... glug glug... stop... glug glug...

Of course I have no idea how we would pay a plumber or what they charge - please help! What it wrong? It doesn't seem like a clog!

OK! The problem is that you have several issues. The fact that it goes glug-glug tells me it's attempting tom vent through the trap which would indicate a blocked vent. Did it always make this noise? How old is your house?
Now let's tackle the drain. You tried everything except to snake the drain and that's what I would have done first. If you don't have a snake pick one up.
Let me show you how to snake a tub from the ove4rflow.
Unscrew and remove the overflow plate,(The one with the drain lever) and pull out the tub stopper. You now have a clear shot at the trap. The way you were going in through the drain it ran you square into a tee that you can't get around. Now feed the cable( 1/4" best or 3/3" with a small tip. You can rent a Ridgid K50 sink cable.) About 2' down you will run into the bend of the trap. Crank and push at the same time to work around it. Your blockage will be found in the lateral pipe going to the stack. You shouldn't have to put out more then 10 or 12 feet. I have found tub and shower clogs to be mostly hair and the worm should auger into it and pull it back. After you snake pour a few pans of boiling water down the drain. This will loosen the grease and flush the mess out into the main. Answer my questions and we'll go on to the vent,Hope this helps and thank you for rating my reply. TOM