I am also procrastinating so I ended up looking through the gallery.
What stood out to me regarding your suggestion was this comment which I snagged off of Wikipedia and which I'm sure you've seen before:
The paths leading up the mountain were narrow and dangerous to travel on. When Zhang Liao wanted to attack the rebels, his subordinates advised him against it because of the hazardous terrain. However, Zhang Liao said, "This is where one-to-one fighting will take place. Only the courageous can advance forward."
With the context being that Mei Cheng had faked a surrender then joined Chen Lan up there. While we're not quite talking gallery road levels of unsafe, it's clearly not the ideal place to do battle and I am guessing there wasn't a giant plateau up at the top to station even Han Dang's men, let alone the whole lot. But I think the main inference point of that anecdote is that, if you have a small number of men, you can hole yourself up some mountain and be relatively secure so long as a Zhang Liao type doesn't come around and start babbling about liquid courage.
I'd be a lot more behind the idea if Chen Lan or Mei Cheng had expressed any sort of big picture thinking. I'm not trying to imply that they were morons, but rather that they wouldn't know what to do if a huge army suddenly fell into their laps. Competent or not, at that point they were just running bandit armies and probably not really considering much beyond the next supply target to feed their men.
Now if these guys flip and
Chen Lan/Mei Cheng quickly realize it's time to come down the mountain and claim some actual land with that infusion of troops, then
you've got a problem on your hands.
But even then...I think the main issue would be such major defections with such large armies hitting Wei/Wu rather than the actual rise of a new major power, I am guessing Chen Lan didn't have the logistical wherewithal to go big time and we'd see another Li Jue/Guo Si drama of sorts play out.
Soon after the battle was over, someone wrote a Fu poem about it.
. . . I acknowledge that this has gotten a little out of hand. But I am going to make this work.
Mainly because it's late and I'm tired. (And still procrastinating.)
Ok so in terms of the original scenario here, you're saying Cao Cao would just leave them alone? I just wanted to give them a terrain advantage, and equal troops, because I thought they'd need it – but you're right, that's not feasible, and it's way more interesting if they go right back down the mountain and start causing trouble. So let's go with that.
There's no question that trying to establish an actual state would be a disaster (these are not politicians), but I don't think Chen and Mei would have
to make a land-grab in order to be a menace. Zang Ba, who just so happens to be here, managed to sustain a (technically) landless, apolitical bandit confederacy for over a decade without being under anyone's formal control.
Regardless, you're probably right, Chen and Mei wouldn't be able to maintain authority. But I don't think that really matters, because even though I think Chen/Mei must have been a credible threat for Cao Cao to throw all these guys at, they're still no match for the level of leadership and charisma of their new buddies, and I don't see any kind of Li Jue/Guo Si battles taking place to oust them. It's actually better (for the bandits) without them in command. (And don't say that they'd fall apart anyway because someone's feelings would get hurt!)
There doesn't need to be a long-term plan here: there's a powerful bandit army on the loose, and they're pillaging all over this part of the border between Wei and Wu, and they've got a stronghold up in Tianzhu. Tianzhu would still need to be taken out, even if it's no longer the main battleground. Cao Cao, or someone
, is gonna have to field some officers here, and I wanna know who's up to the task!
Maybe this was better suited as a what-if than a versus match: would Cao Cao peel off from Jiangling early to take care of this threat? What would Wu do? Alas! Wrong thread. Also, I need a hobby.