As for the cover art and design, authors are often consulted as a courtesy, because nobody wants an unhappy author (or agent), but they should NEVER be given cover approval since that's simply not our job or specialty. We're not art directors or marketing people and, yes, we may well be too close to the work to step back and figure out the best way to sell the book. Not that publishers always get it right, of course, but, trust me, the world is also full of very talented authors who may have very peculiar ideas about cover art.
Ideally, of course, everybody is on the same page. One positive example: the original art for CAPTAIN TO CAPTAIN featured Number One, Kirk, and McCoy down on the planet. When I politely pointed out that the scene in question actually involves Number One, Kirk, and Spock, it was fixed before publication. But I didn't come up with the basic cover concept myself; I was just given the opportunity to look it over before it was revealed.
That's a case of everything working smoothly, the way it should