Hi, welcome to JustAnswer!. This is Ed.
What happened? I'm not sure, but I can only speculate. If at some point what I suggest differs from what your shop has told you, take both opinions at face value and press on.
Your original description of sudden lost power and that the engine wouldn't run unless lots of throttle was given is consistent with either a broken valve spring or a dropped valve seat -- either of which will cause the same sort of problem. Anything that prevents a valve from returning to a seated (and sealed) position will cause a reversion of exhaust being pulled into the intake manifold, polluting the entire engine with low-oxygen exhaust "waste". With reduced oxygen in the intake charge, the injected fuel can't be burned properly and the engine will load up horribly.
This situation is worst when lesser amounts of throttle is given because it invites a greater pressure differential between intake and exhaust. As you add more throttle, intake vacuum decreases and the reversion effect is diminished as you go deeper to the floor. If plugs are pulled, they'll be either heavily blackened or even wet with fuel, depending how close to the actual damaged cylinder they are.
It can get worse, of course. With a broken valve spring, you often have just enough ooomph left in the remaining spring to keep the valve from being hit by the piston. Higher engine RPMs and time are the enemy of a broken spring, inviting the unwanted collision of the two and valve/ piston damage. The valve can actually be bent far enough (or broken off) to stop the piston on its up-travel and stop the engine. Seize it.
A dropped valve seat is actually unusual on the Hemi, but may happen. When a hardened valve seat comes loose from the aluminum head, it rattles around (causing the above drivability problems) and often breaks into smaller pieces that are free to move about the combustion chamber. But because hemispherical chambers have a great deal of room and no "squish" areas (common to wedge engines) I doubt that a dropped seat could seize your engine. It's most likely a bent or broken valve.
To tell, try rotating the engine backwards and forwards at the crankshaft using a socket and breaker bar. A truly seized engine will not move either way. One with an obstruction in the combustion chamber may move backward and forward a good bit until the obstruction once again stops forward motion in the forward direction.
Either way, major engine damage has occurred. A teardown and inspection is the only way you can tell what happened and the actual damage extent. I'm thinking that you'll need at least one cylinder head and piston IF there is no cylinder wall damage. It's a total loss at that point without major machine work.