Small and "in-substantial" are different. If you're selling screenshots as memorabilia, say (posters, tshirts, mugs), then they're likely to be substantial in that context and IMO even with USA's relatively liberal fair use a court is likely to rule against it.

Of course trademark may also come in to play depending on the content of the screenshots themselves.

Your substitution test is useful, and interesting, but it's not a test that determines copyright infringement.

Then you misunderstand fair use. Fair use doctrine is far more complicated than just meeting one of the tests. I used to have to adjudicate Fair Use images on Wikipedia, and believe me the grounds for valid Fair Use is more complex than you appear to believe.