Just a few suggestions to consider for this module and/or future module design. I feel like certain pacing/sequences in the high sewers can get a bit broken. One is resting. After the cult fight in the fire room, you don't get any new resting spots (aside from if you haven't used the lamp or the one in the water passage yet) until you defeat the snare, and then you get another one right before leaving. I did the snare almost last of the entire high sewers. So I got almost no rest until I had completed the whole area, then I got 2 rests very quickly and really didn't even need 1 of them. One possible solution is to just place a 2-rest camp somewhere in one of the rooms and then the player decide when to use them, rather than giving them out right after those 2 specific fights that the player may happen to do in quick succession depending on how they play the level. As far as I can tell there isn't any information provided about any order that should be followed for doing the left side of the high sewers, nor when rests will be made available.
The second is the witches, if you choose to fight them (I think you may have to fight them to get that rod of reversal if you cannot create a fake tome). You obviously don't want to kill off a potential source for trade and magic scrolls prematurely, so it makes sense to decide to fight them at the end of the sewers when you won't need them anymore. But by then the fight is a cakewalk. That's not really a big deal since it was nice to finally get an easy fight for a change. However, the Paladin is then way under-leveled compared to the rest of the party which is a bit of a nuisance. Maybe consider scaling the XP of joining companions to the current average XP of the party, or something like that. Let the player level them up though.