Though I agree with your point for the most part, the Gallade calc is flawed. You are using normal Gallade to calc the damage to both Pokemon, when it should be Mega. M-Gallade does certainly OHKO them both without a doubt.On the surface, the comparison between Keldeo and Infernape is a good one- they're sitting on the same speed, they have similar mixed wallbreaking potential, and deal with a lot of the same counters and checks. On the other hand, this unfairly downplays Keldeo's primary function for many teams- an offensive check to a number of dangerous sweepers and other major metagame threats. Not only can Keldeo's Specs set outspeed and OHKO top tier threats like Landorus, Garchomp, Thundurus, Bisharp, Mega Charizard, Mega Gyarados, Bisharp and others, it also has the bulk to often survive neutral hits in 1v1 situations even when it gets outsped. These situations are where Infernape tends to fail miserably at in comparison:
252 Atk Mega Lopunny High Jump Kick vs. 0 HP / 0 Def Infernape: 291-343 (99.3 - 117%) -- 93.8% chance to OHKO
252 Atk Mega Lopunny High Jump Kick vs. 0 HP / 0 Def Keldeo: 240-283 (74.3 - 87.6%) -- guaranteed 2HKO
252 SpA Life Orb Gengar Sludge Wave vs. 0 HP / 0- SpD Infernape: 298-352 (101.7 - 120.1%) -- guaranteed OHKO
252 SpA Life Orb Gengar Sludge Wave vs. 0 HP / 4 SpD Keldeo: 220-261 (68.1 - 80.8%) -- guaranteed 2HKO
252 Atk Gallade Zen Headbutt vs. 0 HP / 0 Def Infernape: 338-398 (115.3 - 135.8%) -- guaranteed OHKO
252 Atk Gallade Zen Headbutt vs. 0 HP / 0 Def Keldeo: 278-330 (86 - 102.1%) -- 12.5% chance to OHKO
252 Atk Life Orb Terrakion Close Combat vs. 0 HP / 0 Def Infernape: 337-398 (115 - 135.8%) -- guaranteed OHKO
252 Atk Life Orb Terrakion Close Combat vs. 0 HP / 0 Def Keldeo: 277-328 (85.7 - 101.5%) -- 12.5% chance to OHKO
There's a whole lot more examples than this, but those extra base stats pretty much speak for themselves. Even outside of 1v1 situations, just being able to switch into a lot of a common attacks such as Landorus' Earth Power or a random Draco Meteor from just about any Dragon type without immediately getting KOd can help shift momentum and counter threats in ways Infernape can't hope to match. It's arguable whether or not Fire/Fighting or Water/Fighting makes better offensive typing (they both have a number of Pokemon that they each have an easier or harder time with), but I will say that Ground-type moves are so common these days that Infernape is constantly at risk switching into Pokemon like Heatran or Tyranitar that its supposed to checking. Sure, Keldeo gets a Electric and Grass type weakness that Infernape doesn't have to deal with, but those coverage types are so much less common that switching into many bulky attackers is rarely an issue.
I will say that Infernape tends to be just as threatening as Keldeo in a wallbreaking capacity, and Overheat + Close Combat is just as deadly as Secret Sword + Hydro Pump in blasting through the Chansey/Skarm based defensive cores that you mentioned above. If there was no Keldeo I think this capacity alone would warrant putting much higher in the viability rankings, but since it's so overwhelmingly outclassed as an offensive check I don't this is enough to justify raising it's rank.
If you can show me some specific instances where Infernape would be more useful as an offensive check than Keldeo I'd be more inclined to accept your argument.
Infernape has its niche in several spots in the meta, which I think does indeed warrant a rise, even thought Keldeo does indeed do its mixed wall breaking job better.