2F/2F,
I'm the guy. Being the cheapskate that I am, I couldn't resist trying the stuff when I saw how inexpensive it was. Let me address your concerns as listed.
Capacity: Can't really tell you, but I do believe that you'll get at least as much life out of it as you would if you were using it for C-41 films. Truth is, you'll probably be able to run more B&W film through it than you would C-41. With C-41 films you're removing ALL the silver from 3 silver bearing layers. With B&W you're only removing the undeveloped silver from 1 layer. That's why I use it at 1/2 strength, and can push at least 15 rolls of film (a mix of conventional and TMax films) through 1L at 1+9 dilution before it starts to slow down. But be aware that once it starts to slow down, it's pretty well shot. It goes from good to exhausted in a heartbeat. If you replenish simply by topping off what gets carried out with fresh fixer, it lasts even longer.
Shelf life: I don't use it in a deep tank and I bottle it back up between uses, so I can't make a valid claim. I do keep some mixed up at the recommended dilution (1+4) and unused, it keeps well in a closed container longer than 8 weeks. I treat that as a stock solution and dilute it to my working strength as needed.
Hardener: There is none and the normal pH is around 6.5. If you use an indicating stop bath, any of the indicating dye that gets carried over to the fixer will turn blue/violet because the pH isn't low enough. Not a big deal, and it shows that the buffering capacity of the fixer is still within spec. I usually rinse my films with clear water after the stop to prevent carrying too much acid over, and this seems to work. The fixer remains at more or less the proper pH until it's almost exhausted. It's not practical for me to do this with prints, and over time the fixer will start to go yellow as the pH drops from the carried over acid and dye. Not a big deal, the fixer continues to work, though I'm fairly sure that it's shelf life is degraded somewhat with the additional acid.
Temperature: Absolutely no problems using at at B&W processing temperatures. None.
Quality: I don't know if there's anything special about Kodak's Flexicolor fixer that makes it radically different from any other rapid fixer. I think it's stronger simply because it works so quickly. A snip of Foma (a film known to fix out very quickly) film will clear in 30-40 seconds in a 1/2 strength fresh bath. Tri-X clears at about 1 minute. TMax films take a little longer. I've been using it on film and for prints for more than a few years now and have never had a problem. I use it exactly as I would any other "proper" B&W non-hardening rapid fixer.
The only recommendation I'd make is to wash the stop bath off before the first fix to eliminate as much acid carry over as possible. The fixer is buffered well enough to tolerate the carry over, but my reasoning says that its working life might be enhanced a bit if you minimize the acid carry over as much as is practical.
Your wash cycle is very conservative. My wash cycle uses a few changes of processing temperature water with agitation to start. Then, I fill the tank once more with processing temperature water and insert a hose straight from the cold tap and set to just a trickle for 10-15 minutes. Been doing that with films for more years than I care to count, a lot of them with the supposedly hard to wash out acid hardening fixers. I printed up some 40 year old negatives that were handled that way just a few months ago. They're fine.
Feel free to PM me if you have any more questions.