It’s extremely unlikely that manufacturers are processing frozen spinach in different ways. A batch blanch/steam & an IQF steaming tunnel is pretty much standard.
Also, nutritional labeling content for this sort of thing is fairly standardized. It’s not like companies are sending out there frozen spinach for nutritional analysis. It’s basically the same regardless of supplier
No. It doesn’t work like that. If anything, variability may relate to where it was grown and the soil quality, but there’s not really any way to know that unless you have a food science lab set up in your kitchen.