It removes organic material and such out of your aquarium, Its only really needed in saltwater. The only thing I can see needing it for in freshwater is the occasional bio film on the surface of the water but a simple air pump to agitate the water takes care of that easily.
No, it won't. They really don't function on freshwater tanks. If your tank smells I would cut back on feeding, increase water changes, and consider adding carbon.
Have you checked water parameters? I would try to figure out the cause of the smell and get rid of it that way instead of adding something extra to get rid of it
Protien skimmers work in some setups with freshwater, like ponds. All it would accomplish in an average freshwater tank would be hyper agitate the water, possibly increasing oxygen content, maybe. If you add salt to your tank, like may people do, than the protein skimmer will work to a certain degree, not enough to justify the cost of one though.