If you're writing the contract, it can say whatever you need it to say (within reason, of course).

How about
1) revising the way you present milestones so that it's clear that you won't start work until on X until you've received all relevant materials? You'd have to include some kind of acknowledgement of the wavy final deadline if you do this, though.
2. including a review period (5 business days, 36 hours, etc) to make changes or submit copy, after which you assume the work is satisfactory (if relevant) or the section is moved out of the contract and subject to re-negotiation. How this would work and how well it would work would depend on the structure of your contract and on the nature of your business, of course.