Maybe I'm not understanding the problem. When you refer to the sear, I take it you mean the DA sear that is pinned to the front of the hammer and has a light spring behind it to return it to forward position when the action is at rest. You can check the spring on that by pushing the lower end of the sear back against the hammer. If it does not return forward, the tiny coil spring is broken or missing, and you need to replace it.
What I don't understand is this: without the spring, pulling the trigger should just let the upper trigger spur ride right past the sear, turning the cylinder but not moving the hammer. If you cock the hammer manually, the DA sear is not involved at all and should not affect your ability to put the gun in battery for SA operation.
You would also get a lock up of the sort you describe if the hand was jamming on something, or if the cylinder was blocked from turning. Is the action free if you swing the cylinder open? (Holding back the bolt with the thumb release, of course, so that the hammer is not safety-blocked.)
I always wonder about small crumbs of metal that get down in the action and prevent something from completing its full range of motion.
I don't see anything in the photo that looks obviously wrong.