Young adults in the early sixties were very politically involved, and so they wanted to listen to the music expressing controversy and hard times
Why were traditional folk musicians offended when Dylan began to use amplified instruments?
Because he was changing tradition and many people don't take to change well. Also folk is about the words not the music as much and amplified instruments can take away from the words.
Is acoustic folk music more “authentic” and folk-rock too commercial to also be heard as authentic?
folk-rock is more commercial yes but that doesn't necessarily make it less quote on quote "authentic."
Frank Zappa criticized singer/songwriters for being too personal and too quick to dump their problems and feelings on their listeners, who are already burdened with their own problems. Is the widespread popularity of music by singer/songwriters an indication that he was wrong, or did he make a valid point?
I don't think he has a point, the reason people listen to them is because they relate to their problems, for example, no matter what the problems in your love life are I bet you could find a Taylor Swift song that makes you feel like you aren't the only one who feels the way you do.