To my English ear, when I hear "slumped over" I picture a person laying their chest and arms over the top of something, like a desk or a fence or even the handle of a shovel. You generally "slump over" some object in a lazy way. To "hunch over" or to "hunch" means a to be bent over in the same sort of posture as "slumped over" but you are not leaning on anything. A person can walk down the street, or stand at a bus stop "hunched over" in a bad posture. And yes, you can use "slump" all on it's own, but in that case you aren't being literal. For example, "I hate my job. I go to work and slump through it every day". This gives the impression that your work is draining and tiresome, that you need to rest on something all the time just so that you'll have the energy to finish. Of course you aren't literally "slumping over" something all day long, you're just giving that impression.

'Slumped over' is an adjective that describes a certain position. This position is one where the person is in a sitting or semi-sitting position, and the upper body is leaned forward over something, such as a desk or table. It is usually used in situations where the person gets into this position involuntarily, such as dying, losing consciousness, after a seizure, etc. 'Hunch' is a verb describing the lifting up of the shoulders (not the arms, just the tips of the shoulders) and arching the upper back forward. Picture someone who's trying to avoid security cameras that are perched above. He would walk with his face down, and with his shoulders rounded upward (so as to further hide his face). Another example, picture the Hunchback of Notre Dame.