A single Redgaurd will defeat the largest dragon ever to walk on Nirn?
Are you kidding? He wouldn't stop there... He'd brandish the head about and ask, "Who's next?"
Remember, pissed off redguards are like steriods on steroids. Especially when they are fighting for their homeland... And in the time it takes for the Ka Po'Tun to recover from their leader's critical existance failure, it could give the resourcefull khajiit enough of a foothold to secure the demise of the KPT... And the KPT are more like Dunmer without the fire resistance than Khajiit... Just because they have fur does not make them comparable to the Khajiit.
The Tsaesci have nothing in common with the argonians other than the scales... They are IMMORTAL! VAMPIRE! SNAKEMEN! not tree-licking newts.
I think it would be best not to focus on the unlikeliness of a complete KPT victory... Though I've been focusing on the Khajiit and Redguards (my two favorite races), none of the other races are pushovers either, with the possible exception of Imperials, who's special power seems to be primarily to take the blows that would otherwise go to the more important characters.
How would the mighty armies of the KPT react when they invade Skyrim and find three companies taken out because a certain Nord couldn't shut up...
Or the Altmer Psyjiics using their knowledge of magic to pretty much will the KPT army out of existance?
Or the fate of the KPT who hit a Bosmeri tender spot and end up on the menu for the Wild Hunt?
Or in Morrowind, the KPT choking to death in a storm of Ash and fire, while the Dunmer wade through the disabled armies with impunity...
Or those that dare venture into Argonia, where the trees decide they don't like these new kitty-cats? Or an invasion of High Rock, where they find themselves cut down by mounted Breton Knights, who upon being faced with a relentless barrage of KPT magic respond, "Stop it, that tickles!... My turn... HADOKEN!!!"
They would also be unwise to underestimate the other race native to high rock, as a bunch of super-hyped Orsimer remind the KPT of the drawbacks of Medium armor, and laughing because their armor is better, cooler, and just as mobile.
God forbid any enterprising Redguards or Dunmer decide to use those exotic relics left by the Dwemer, and possibly overpower the KPT hordes with moar dakka and humongous mecha.
I guess the Imperials could use their Voice of the Emperor ability to convince the KPT they'd be better off fighting others... or just turning their boats around and going back to Akavir... Or, if they are feeling particularly fiesty, halfway through a major battle, the formerly exhausted Imperial Legion suddenly strikes with renewed vigor when the front lines of the KPT find themselves passed out in the middle of a battle...
To lay on more... If the sload decide they hate the Akaviri more than they hate the Tamrielic races... Then in addition to Hammerfell's bad-asses, Skyrim's loudmouths, High Rock's Knights, Elsweyr's battlefield behemoths, Dunmer fanatics, Altmer super-wizards with game-breaker powers, Orcish invulnerable berzerkers, Bosmeri horrors, Scavenged robot armies, Hist bio-weapons, and the possibility of the Imperials pulling another Dragon out of their ass, the KPT will find themselves facing deathless Marines as well.
This could make an awesome RP... even if it does get recorded by Akaviri scholars as "Disaster at Hiroshima," because they make the exact same mistake Uriel Septim V did with his invasion of Akavir.
It may very well be that rather than the Imperial infighting making it an almost impossible situation for the empire to instead just hold back the forces of Tamriel from completely roflstomping the n00bs all the way back to Akavir.