I have almost no experience in helping adults learn to read. Does anyone know if adults may be less likely to jump to a conclusion about what a word is?

I don't know about adults, but one of the high-schoolers I have in an SAT-prep class does this continually when reading aloud. He seems to follow the "see the beginning and end, guess the middle" approach you mentioned, reading "miserly" as "miserably" and such. I'm not yet sure if he only does this when reading aloud (the added pressure of being "on stage" makes many normally good readers horrible at reading aloud), but it could be a big liability for him on the test if he does it when reading to himself, as well.