Agree with you completely on the lack of scouts. Any decent army would have outriders, scouts and pickets at least 10-20 miles out. The Lannisters should never have been caught like that in the field. As King Robert once said, only a fool meets the Dothraki in the open field.
It's even harder for a fleet to fall upon another by surprise.
And I agree about House Tarly. They were bannermen to House Tyrell. I don't recall anything about Cersei promising to make them the replacement House for Highgarden, but maybe that happened.
Still, if everything was logical and militarily sound, it would be a much longer, more boring show.