The screenshots didn’t help me much since everything was blurred out and it was hard for me to mentally map how everything was related. I think I’m also missing how the followers are attached to users. But if I understand what I think you are asking, you essentially want to create an array of all followers from all users who have written reviews for a location. You can easily create the relation from Locations to Reviews, do a lookup of User name, then create a relation from the lookup to the User’s sheet. I think where you run into the problem…and I’ve seen this before…is that you cannot create a lookup that creates an array of arrays together.
I posted about this situation awhile back while helping somebody else, but it never evolved into anything beyond that. Maybe this would be a case for a feature request.
As an alternative, maybe you could try to think backwards and create your relation from the users to the reviews sheet, then a lookup to get the location and then create a direct relation from location to users. Again, I think I’m missing something to completely picture this. Also, kind of mentally out of it today.
Also take a look at this to create a default value to use for a relation only when the signed in user is following a user who review a location. I think you could set it up so an if/then colum will only be filled, possibly with the signed in user’s email, when the signed in user’s email is found in the follower’s array (might need a template of signed in user’s email, to a relation in the followers array, to an if/then column that checks if the relation is empty or not and returns user email). Then you could use your original thought or some of the ideas I presented to build your email array on the locations sheet, but it would only contain the signed in user’s email for the relavant locations they can see.