Just a few comments, I played it in a party of 4 all the way through.
1. On the Shark - We did do this without killing the shark. My character was a Enchantment Wizard and I used hypnotic gaze on it and we took the things off of it. I did not realize the wall was supposed to be knocked open. Our DM presumably improvised, when it became clear we were going to strip the incapacitated shark and flee he had everyone roll a Save (wisdom or charisma can't remember). One of the characters got "possessed" and opened the secret passage to find the Mcguffin. I assume it was all ad lib. That character had a vision or something, the whole time I was keeping the Shark incapacitated. Eventually he came out of it and we fled.
2. The rivals were pretty good and they had different relationships with different characters in our group. Our Half Orc Barbarian was pretty close to Maggie in particular. My character (Dwarf Order Cleric/Enchantment Wizard) was a bit standoffish. That dark haired woman (Elf?) disliked our whole party. But overall I would say they were allies. We did come to a standoff at the very end of the campaign. They started to fight to prevent us from going into the last area. This was the toughest battle. I was held early and when I got out of it I was ready to go postal and just lay waste as kind of a "you dare attack me!!" response. I downed Maggie in one shot, then our half orc stabilzed her! Their Goblin also healed her to get her back up. Their Elf kept fighting, shooting me with her bow. Then our Bard called on everyone to stop fighting and talk it out, the DM kept us in initiative, a bunch of redied actions, "if anyone attacks I do this" and a standoff going around with calls for the other side to stand down mixed in. Eventually it got to their guy that looks like Matt Mercer and he called for us to surrender. At this point the Gob, Maggie and our half orc were not fighting any more, their Elf still was, our Bard and Rogue were waiting to see. I still wanted to kill everyone, but I responded to Mercer's call to surrender with my own demand, but I backed up my request with magic - I upcast Command - "Surrender" and hit everyone of their guys with it. Maggie and the Gob were willing anyway, the Elf who wanted to keep fighting failed her save broke her bow and stormed off. I don't remember if Matt Mercer failed his save or not but he did surrender. Then they left and we went in and fought the big bad (or was he a big good). In any case he was not very hard.