Does this message apply to every bug mentioned in this thread, or just the OP where they couldn't complete the act?
It was primarily directed at error3, but in general for any issues that are not already known and may not be easy to reproduce, it would be nice to have an applicable save for.
It is suppose to take a few rounds for the worm to spawn. Alexander will fight the worm, and vise versa, but target selection depends on various factors, including positioning.
From his perspective, you are the reason the worm is there in the first place, and given what he knows about what you know about the fate of sourcerers on the island, he has reason to distrust a temporary alliance.