In addition to the two rules (depending on natural pronunciation) of adding an apostrophe to a noun ending with the letter 's' here on OxforDictionaries, the apostrophe is not added to a noun that already has an apostrophe.
Having said this,
Wendy's product manager insisted...
will serve the purpose and is grammatical.
A couple of examples -
McDonald's chief executive Don Thompson defined under performing Australian and Japanese markets as weak and said the Illinois-based company was concentrating on 'stabilising' key priority markets in both Australia and Japan, as well as Germany and the U.S., reported the Sydney Morning Herald - from DailyMail.
The McDonald's parking lot is full of pickup trucks with fishing gear jutting from their beds, sleek hybrids with kayaks or mountain bikes racked to their roofs or tailgates and family vans packed with kids coming from soccer tournaments - from Tribune.
And finally,
In that same interview, Wendy's chief marketing officer, Craig Bahner, noted that Wendy's customers widely request the pretzel bun for other products. Sure, it costs the customers 30 cents more, he said, but "we totally accommodate. We want them to have the product they want." - from USA Today
A Google search for 'Wendy's chief marketing officer' gives the result with the answer Craig S. Bahner.
A little note: I think when we talk about some authorities from some company, not putting an apostrophe is okay. In other words, when we talk about the designation, apostrophe is not mandatory to use. For instance, "Microsoft CEO said that..."; "IBM spokesperson added that..."; "McDonald's Marketing Manager agreed that...", and an article with the headline and an image caption from NYDailyNews which reads Former Microsoft CEO Steve Ballmer etc.
A noun is often used as an adjective to indicate something of that "type". For example, a "car part" is a part of type "car", that is, a part that is intended for use on a car.
A noun used as a possessive, of course, indicates that the thing "belongs" to that noun in some sense. A "car's part" is a part of that car.
Often the distinction between the two can be subtle. "I found a dog collar in the park" -- I found a collar of the sort that is worn by dogs. "I found a dog's collar in the park" -- I found a collar that belongs to a dog. In context they both mean pretty much the same thing.
But usually the possessive is more specific. We say, "This store sells car parts", that is, they sell parts for cars. We'd be unlikely to say, "This store sells car's parts", because that would sound like they sell parts for one particular car. But I'd say, "My car's steering wheel is loose" rather than "My car steering wheel is loose", because I'm talking about the steering wheel of one particular car.
In this case, it's common to say "the morning news" to refer to a TV news program that is broadcast in the mornings, or such programs in general. "This morning's news" means something that was in the news this morning, one particular morning.
"I heard about the hurricane in the morning news" -- I heard it on a news program that is broadcast in the morning. "I heard about the hurricane in this morning's news" -- I specifically heard it this morning. In context, the difference is very subtle.
But, "I like to listen to the morning news" -- I listen to it regularly. You wouldn't say, "I like to listen to this morning's news", because you really could only do that once.
Best Answer
Yes, the benefits of the product = the product's benefits. The disadvantage of the product=the product's disadvantages
No need for plural, as it is a general statement about a product.