Uh, ok. For one this is a Rankurusu thread but I'm going to address these points.
Payback is not definitively better than Stone Edge. It's not even arguably better. What do you hit Payback with? Ghosts and Psychics. What Ghosts do you encounter in OU?
Shandera: OHKO either way
Gengar: OHKO either way
Burungeru: Bulk Up fodder. eventual OHKO with either move.
And that's it really. What Psychics do you see? Doesn't matter, you should always switch against Psychics that pack STAB Psychic like Rankurusu, Espeon because they will kill you. Now what does Stone Edge hit? Fire, Flying, Ice and Bug. Those are two types that resist fighting like Payback, but Stone Edge hits Psychics and Ghosts much harder than Payback hits these. Tell me, with Payback how exactly to you kill Ulgamoth, Gyarados, Zapdos, Dragonite, Salamence, etc.
The coverage and power are simply better, as is the crit chance. The only thing is the accuracy and you can use Rock Slide if you REALLY want to. Point is with Roo's stats and the current metagame Rock is better than Dark for coverage.
As for the above two scenarios: Why would Gliscor Protect stall you when it can Acrobat? And if it doesn't have Acrobat then a smart player would Bulk Up on the Protect turns and preserve Stone Edge PP until you have enough Bulk Ups, like 3 or 4. Yes you can Taunt but if you poisoned him and he used Bulk Up on the switch that's still a +2 Stone Edge with a high crit ratio. It isn't certain you'll out stall that.
As for Toxicroak, it may resist Fighting moves but Roopushin can beat it with brute force. Toxicroack isn't exactly bulky and even one BU will give Toxicroak significant trouble as it keeps getting it Subs broken and never gets a chance to Sword Dance.
Now, as for Rankurusu finally vs Roopushin, even if most Roopushin invest in they're SpD their Df is naturally higher and get's raised by Bulk Up every time AND Psycho Shock has less BP. If you're coming in on a Roopushin that has a +1 or +2 under it's belt and you try Psycho Shock you actually won't kill it, unlike Psychic which always will. Considering Roopushin is #3 in usage and Blissey/Chansey are somewhere in the 40's at least, which threat are you going to prepare for? Plus, as was stated ALL Blissey can do thanks to Magic Guard is Seismic Toss or Wish while you CM to +6 and Recover the damage.