I don't have a lot of experience with Fetta, and I know from your posts that you are an experienced cheesemaker, so please don't be offended by my suggestions
Cheap store bought milk: From reading the posts here, it seems that there is a lot of variability in the store bought milk you guys use in the US. Have you used this brand before and was it the freshest milk you could find?
Homogenised milk often has a much softer curd due to the mechanical action of the homogeniser on the milk proteins. Also, store bought milk is often pumped around the factory a number of times before it gets to the bottle, damaging the milk. Finding the best quality, freshest, non homogenised milk is key to getting good curds.
I presume you have been using this rennet for other cheeses and had no problems?
Your stirring regimen looks good, should have produced nice firm curds, without too much moisture
You stated that you cut the curds to 1cm cubes, but your photo looks like the curds have broken up a lot during hooping, indicating soft curds.
Brine salt content. I usually use 12% brine, which really firms up my curd and provides a good shelf life.
In summary, I would suggest that your curd has been quite soft, due to the milk you have used, and then the brine has been too low in salt to set the curd. The photo of the loose curd sitting in the whey/brine shows that the curd is just breaking up. The fact that the tase is good indicates to me that everything else is fine.
Hope these thought might prompt some further ideas/clues.