Can an Open Hand Monk push/shove a Banshee using Flurry of Blows, in spite of all of the banshee's condition immunities and features (i.e.: grappled, prone, hover, incorporeal, etc.)?
Yes, you can push the banshee
There are no general rules for incorporeal creatures that make them entirely immune to physical interactions, the banshee for example only has resistance to physical damage, so it clearly can be interacted with. The incorporeal movement trait of the banshee only allows it to move through solid objects. The condition immunities it has to express its incorporeal state only make it immune from being prone, grappled or restrained, not from being pushed.
You will not be able to knock it prone, as it is immune to that condition.
Open Hand Technique (PHB p. 79).
Whenever you hit a creature with one of the attacks granted by your Flurry of Blows, you can impose one of the following effects on that target:
- It must succeed on a Dexterity saving throw or be knocked prone.
- It must make a Strength saving throw. If it fails, you can push it up to 15 feet away from you. (...)
Condition Immunities Charmed, Exhaustion, Frightened, Grappled, Paralyzed, Petrified, Poisoned, Prone, Restrained
Damage Resistances Acid, Fire, Lightning, Thunder; Bludgeoning, Piercing, and Slashing from Nonmagical Attacks
Incorporeal Movement. The banshee can move through other creatures and objects as if they were difficult terrain.