I think part of the problem was that there wasn't a whole lot of content with the player character for them to draw off. It was pretty much just his initial quest and then a bunch of sex scenes, making it about his only piece of major content (helping him deal with his teacher) makes a fair amount of sense. As for her being redeemed, eh there was a throwaway line about how the relic made promises to her so I guess the idea was that she was being influenced? It was kinda weak but I've never been one for going full doom marine in these games so I'm not that fussed.
I honestly disagree with their not being enough content to flesh Berwyn out. In most of Berry's interactions with the Champion and much of his flavor text with other companions its established that he loves magic,reading and wants to go on adventures (which is why him just retiring is weird to me). The other thing that is made clear in one of his dialogues is that he becomes frustrated once he realizes that Mathila had been purposely weakening his magic and teaching him improperly. This establishes that Berry is not that strong of mage, which is once again kind of goes against him suddenly being this god tier mage at the end. It also sets up that Mathila had some alternate plan in the works, but well probably never know the direction that story would have taken as the original writer left. The cuddlefuck scene also shows that Berry can be tender with the champion and their sex life isn't purely dom or sub.
In fact I'm pretty sure the writer said Berry's more romantic path is him learning to let his guard down around the champion, as Berry has trust issues from the events with Mathila and his parents, and he would be willing to becoming something different if lead to his partner accepting him. I think this is part of the reason he's okay with Mathila slowly changing him into a girly boy, as he clearly trusted Mathila at the beginning and probably accepted it to make her happy.
Pretty sure it was B who said this as it was them defending Berry getting a gender swap. I also believe he came to this conclusion after talking with the original writer. But take what I'm saying here with a grain of salt as I can't find the quote and don't follow the development closely enough to be sure.
Point I'm trying to make is if given enough room to grow a lot of Berry's arc could have been something really interesting without destroying his established lore. Him being a sincere lover who supports the champion through their struggles or femboy that doms the shit out of the champ could have worked. There is a potential arc for him becoming a god tier mage that surpasses his master. It could have been meaningful if he was given proper attention and time to grow into it. But it feels rather rushed. Which from what I've been reading here about the games development makes sense.
Now don't take this as me dumping on B. Like FetishGirl Pointed out B had to account for multiple variables based off the potential transformations. B put in all this lore and tried to justify Berwyn's character in this quest. Considering what B had to deal with and the fact that he did not really want to write this character to begin with. I'd say he did a good job.
Now what actually does bother me about this game and makes me a little irritated is this game has potential. It honestly remind me of Mass Effect Andromeda or Mass Effect 3. Both have an interesting premise but for one reason or another they feel apart. Now while Mass Effect had a shitty publisher that was constantly micromanaging and got in the way of artist doing their jobs. CoC2 seems to have shitty management team that has no type of structure and randomly introduces new shit with no real type of plan what so ever.