I tend to agree that, if we ever do find out her alignment, it will probably be chaotic evil. But to repeat my above post in response to your other points:
- First, we don't actually know Calder is evil (although I suspect he might be given his demands for servility - ironic since Serini has forced him into servility for so long).
- Secondly, its questionable whether being evil alone is grounds enough to imprison - some would suggest you should only be judged for your actions (and then by someone with authority to do so).
- Third, Calder is not merely imprisoned, he is enslaved - this is not a prison, instead Calder is there to perform a job for captor (guarding the dungeon).
- Fourthly, Serini effectively tells us in strip 1283 that she is not righteously locking up evildoers, but forcing monsters into guard duty because it was expedient (she contrasts it with her current approach of getting agreement from vulnerable monsters to do the job).