I'm pretty new to these Blend Shapes so I'm a bit lost on how exactly they work.
I've started with importing a file that I know works for others as I've seen it work for myself. With the file came the six expressions and the original mesh itself.
My issue is in the Animation Editor > Blend Shape. In this window I'm seeing every piece of the 6 expressions laid out to key (like the top left eyebrow, bottom right jaw, etc.) I'm aware that I'd have to combine the meshes into one final mesh and name the expression but when I attempt to combine anything I'm receiving the error: // Error: line 0: Invalid selection : polyUnite needs at least 2 polygonal objects.
I'm sure you know where I'm going with this but If not, I need the six expressions like I have shown in the 3rd screenshot, not each mesh like shown in the 2nd.
I am not sure if you should combine any mesh for using blend shape function, as this function only works as you have the equal number of vertices in both blender shape (like the top left eyebrow, bottom right jaw, etc.) object and the original mesh (the base face which is all the shapes been added to) object.
All you have to do is apply the blend shape function after chronologically selecting each blended shapes and finally the original base mesh by going Anim Deform > Blend Shape > OPT  and make sure the deformation order is set to front chain, this is a process of adding target, also typically the controls you have mentioned which are found Windows > Animation Editor > Blend Shape will only work when the target/blended shape with same number of vertices and not combined at all is ideally set to the base mesh.
“We’re all one thing, Lieutenant. That’s what I have come to realize. Like cells in a body. ‘Cept we can’t see the body, like fish can’t see the ocean. And so we envy each other. Hurt each other. Hate each other. How silly is that? A heart cell hating a lung cell.”