“Brother,” said Posbrake, “why has our alliance acquired a third side?”
IMO makes sense either way.
Side A makes alliance with side B, then side C.
All three sides are now allies with same turn, don't attack each other.
If Side A breaks alliance with side C, is side B still allied with side C?
Yes and No both fit with above, and not really important as side B and side C which never before made an alliance could easily make one or break one if it mattered later.
Because Posbrake explained plan sounds stupid, imo either he has extra secret plan, or he is likely under influence of carnymancer (weaken one stat/rule).
[Plan to attack self makes little sense because it weakens side in case someone else is still going to attack. But it makes sense if trying to get someone else overconfident and 'break an alliance and pay penalty'. The original prediction might have identified that Creen would betray Posbrake and all this was setup to trap Creen for example, make him pay penalty for breaking alliance, then capture or croak him, and use money from breaking alliance to hire way to defeat mother side.
As far as long time customer goes, Posbrake wanting to hire a Carny, Posbrake not backstabber (if other side backstabs first, not really betrayal), and wanting to 'fight fate' would make him seem a nice choice to survive for Charlie and friends... customers that don't backstab are less risky even for a backstabber.]
It still also makes sense that Posbrake would be legit sad when Creen reveals backstab... means everything is now certain, no hope my brother was better than that. It would be like a parent that heard rumours that son was drug addict and planning to rob parent at gunpoint for cash... once it actually happens it is still heart ache for parent, even if parent has trap set up (gun won't work and police hiding around corner) just in case.
If Creen made contract with Charlie first, then Posbrake secret plan may fulfil obligations... Charlie has already taken out air defenses, and remaining obligation to attack Posbrake if he doesn't comply could easily be handled with "no ranged attacks can hurt me". Creens force is all archers and Creen. So may be only one guy (Creen) verses Posbrake+dirtomancer+Carnymancer+Golem+chief warlord... croak Creen, back everyone else off and archers autoengage king with ranged fire while king picks them off one by one and levels.
Even original predictomancer hiring could have been "will Creen and friends betray me, and if so how?" rather than more generic question. Predictomancer's work best with specific questions.
Creen wanting to see books may have been Creen wanting intel to make sure his betrayal plan would work.