I would say that the first two sentences are interchangeable. They mean precisely the same thing. There is a slight nuance of difference between the second pair of sentences. "Fingerprints help identify people is like a statement of fact, a truism, something you might learn in a science class. "Fingerprints help us identify people" may mean the same thing as above, but it may also be referring to a particular situation. For example, someone in forensics may be talking about the significance of looking for fingerprints. A bystander or student might ask, " Why do we look for fingerprints?" The instructor might respond, "We do this because fingerprints help us identify people." So in the first case, there is no 'participation' implied. In the second sentence, active participation in using fingerprints for identification is suggested.

Yes. Exactly the same and interchangeable. Good job.