That latter part is to prevent resale, which is not related to retail price and manufacturer's wholesale price.

It depends on the buy back price for if this scheme benefits customers or not. Lets say I'm nabisco and I tell grocers that instead of putting my product on sale and potentially take a loss that I'll buy back that product. The risk of selling the product is on nabisco rather than the grocer and the grocer may stock more nabisco goods because of it. However, if the terms are not favorable then consumers should be more skeptical in agreeing to those terms depending on their risk tolerance. Extreme examples of this do often fall into anti-trust.

Just a short time ago, there was a thread where someone posted a link to the infamous story about Vlasic (IIRC) and the extremely cheap large bucket of pickles that Walmart (the retailer) forced Vlasic (the maker) to provide. The store mandated the maker. Not the other way around. So yes, sometimes the grocer does the telling.