In theory. The only compulsory quests are:-
A Timely Rescue
Amalgamation
Allies Among Ashes
Betrayal at Fort Ikon
Necropolis
Bane of Cairn (To proceed to the next difficulty)
Also, you need to kill Cronley (to access Twin Falls) and Karroz (to access the Valley). But you don’t need the quests to do so.