Indeed, and that is a good Get The Band Back Together argument.
Simply put, not, it is not likely at all. The nature of D&D combat is that you're going to lose ressources unless you can trivialize the encounter with your at-will capacities only without taking any damage.You mention that that the winning party might lose resources in the fight - which is possible. But more likely they would gain resources.
Calder doesn't seem to have any magic items, so at worst he could gain the items of the intruders. In any case, removing the intruders who wield them is still a net benefit.It's been suggested in this very thread that the Order might be fighting Calder to gain a level or two, and the victor (out of Calder and the invader) may gain magic items.
As for gaining levels, from what we saw in the rest ogéf the comic, gaining levels in the OotS-verse doesn't heal you or give you back spent powers immediately. Maybe the survivors of the clash would be a level higher, but they'd still be hurt and out of juice.
Note that the worst case scenario is "one side enslaves the other after trivializing the encounter".Or indeed, as you say, the worst case scenario could happen with one of the two groups enslaving the other (either through Calder's mindbending, or Xykon's zombiefication).
If potential intruders enslave Calder but have burned through all their 9th level spells and lost 1/4th of their total HPs, that's still a net benefit.
Calder escaping without slowing down tge intruders is a "problem for later". Serini probably thought he could be tracked down and killed this time if he was up to his old tricks, and it'd be a greap trip down nostalgia lane for her team. Maybe she even thought that if they had to work together long enough, they'll remember they were friends once.Indeed, in one of the other threads someone (I can't remember if it were you or not) suggested Calder might be able to avoid a fight - he might ally with the invaders, or simply avoid them - so now there are two threats, or the two threats have combined.
Calder joining forces with the intruders without a fight has no chance to happen, based on what we know of his personality.
As said above, best case scenario is they take each other out.If Calder wasn't in the dungeon, the worst case scenario would be that there is one threat there. With Calder in the dungeon that is the best case scenario.