Re: tub draining - as the line slowly drains, it leaves room for more water behind it. That's why the tub drained. Had you flushed first, that probably would have drained, but then, not the tub.
Anyway, clog is past the cleanout. My first thought would be, "why the hell is the cleanout open??" Any enemies? Anyone mad at you? Pouring a bag or two of concrete mix down that pipe would be pretty easy. And, due to settling, it wouldn't completely block off the pipe. Or, perhaps kids unscrewed it and shoved a ball down there (that barely fit).
Myself - I'd attempt snaking it first (with my own snake) - only because that solution is free. If I couldn't get it, it would be pro time.