80000 riders? Where would such numbers come from, after the constant wars of the Khans? Anyways, it will certainly be a challenge. Remember though, those 100,000 levies are levies, not hardened steppe warriors. So we're talking about a strong force, not an unstoppable tide. Eighteen Tumens? Then I'll be worried. But the Byzantines can field nearly a million men if they were to mobilize en masse.
And the quality of the thematikoi has risen greatly due to the constant infighting, which caused the princes to create their own professional forces not unlike the Imperial Tagmata. I'm not entirely sure if Nikephoros means to take Italy, or what, but if his "crusaders" do make it to the East, they would at least bolster the forces that initially assemble, and be rather useful bow fodder, if nothing else.
I was never really impressed with the Mongol's performance before, and I am still rather unsure as to whether they can really threaten such a large and well established Byzantium. One of the main reasons the mongols were able to sweep across the earth was that they faced small micro-states fighting each other, for the most part. They were a newly unified force in a sea of squabbling peoples.
The various nations in China, India, and Persia never truly united against the mongols, and the Rus were practically in a state of civil war at the time of the Mongolian Invasion, and the Turkish states in Anatolia were also quite divided, though the Mongols did finally grind to a halt by then. In fact, I'm not really sure why the Romans haven't decided to take back their northern possessions.
The MAGYARS stopped the Mongols, at the height of their invasion. Magyars that would have been a mere nuisance to Byzantium. It seems that the malaise that sunk into Thomas II after his ridiculous charge at Neapolis sunk into his people as well.
Also, what has become of Sortmark and the 'Kingdom of the Rus'?