I'm now a little bit lost. Originally you said the problem was ink coverage, now you say both ink coverage and resolution. These are separate issues.
Your ink coverage issue is a matter of using the correct output profile. That SHOULD BE supplied by the printer. In fact, they really should supply a .joboptions file to define ALL of their requirements for a PDF. Shor of that they should at least send you a real written specification.
Your resolution problem could be with the cover image being scaled, or with some other image if you have others. Can you show me a screen shot of the Info panel withthe cover image selected, please. You have not reduced the resolution below requirements for offset printing in the export settings, so if there is really a resolution problem it is in the images themselves.