The point is to tell an exciting story - there's no right or wrong definition of what that means for you.
The dice's purpose is to take you down paths you might not have chosen deliberately but the goal is still to have an exciting story. If the DM wants to be like "I recognize the dice have made a decision but given that it's a stupid ass decision, I've elected to ignore it" then he has my full support.
Maybe a cleaner way would be to decide up front: which outcomes am I ok with? and simply cap the roll at that. You know the paladin only has 17 HP left and you don't want the paladin to go down so the maximum roll you want is 16. So if you have roll 4d6 damage. You do: roll 3 roll 8 roll 12 roll ~~18~~ 16.
Yeah definitely get the Atomic Bonds expansion. The competitive basegame is ridiculously broken. For example there are quests that send your all over the map. But the silly thing is if a quests needs to be "picked up" in one location and "turned in" in another then once it is picked up ANY player can just snipe the rewards if they're close to the destination. Also many exploration options are hillariously random. This makes the competitive version little more than a crapshoot. It's frankly mindboggling how anyone actually greenlit that design.
In coop all that is still the case but instead of being frustrated when another player gets an extremely lucky pull in the exploration, you cheer each other on. Sniping different quest stages between multiple players becomes a cool strategy instead of a braindead design choice. Most of these anoyances fall away and you can revel in the setting of the game.