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Hello Daniel, I would not represent 404 as that indicates a broken url to the search engine and it won't know why. i would not use noindex as the page should still forward pagerank however it has no content. I think when there are not any reviews I would simply add some more text to "There aren't any" like "There are not any reviews yet, you can look XY business details here, or you may check back at a later time. If you have your opinion feel free to add the first review yourself". "Xy business details" can be a link to business detail page, so there is some anchor value carried. The page still has the ability to rank in google an to be found if you search xy business reviews. From engine perspective this is bettter than redirecting or 404, and still makes sense from user perspective as you offer them opporunities to get engaged with your page. Maybe I would do it that way.

The only concern I have about getting to granular is time. Things like the 80/20 rule come to mind. You don't want to chase down minor details when there are big fish getting away. With that said, there is a lot of traffic involved with plural vs singular keywords. Your time is well spent!

Thanks for the feedback, there may be better places to allocate that budget. Due to the nature of the site it doesn't collect many registrations at the moment, it's mostly people coming in off of search, finding what they need and leaving. Building out a better funnel for converting visitors is the next development task after the multi-tenancy is fixed.