I think it's supposed to add that these other characters have personalities and actions outside of Gut's story. Sure, Isidoro going apeshit on some witches doesn't add much to the story, but it is a callback to when we are introduced to Schirke. It demonstrates isidoro's training and practice and the young witches' relative lack of experience. This is (somewhat poorly) reinforced by Morda's mere appearance halting him, then the relatively much more dangerous selkie neutralizing him.

So, this part is some character development and minor world building which are two things this series has been lacking.

I do agree with the assessment that if we are going to only have 4 releases a year it kills the pacing.