Hi - welcome to the forums!
Re your first point. If we make the subsidiary assumption that humans tend to categorize certain objects into classes, where the objects in the class are all of equal serviceability from the point of view of the actor, then we have our definition of a good. We can do praxeology in a world without this assumption, but it wouldn’t be very interesting. People really do think this way though, so we might as well study action in a world where this is true, rather than any other possible world.
I’m not sure what you mean. Do you dispute the assumption (in italics) above? Do you always think of the pair of milk bottles in your fridge as two entirely seperate items, like milk and cheese, or do you actually group them together and think of them as “my supply of milk”?
Re your second point, I don’t see where the problem is. The ‘optimal size’ of a firm is determined by “economies of scale” and “diseconomies of scale”. One of the diseconomies of scale is the inability to internally calculate.
Your third point was discussed here recently. I will have a look for it.
[EDIT: Here is that discussion: Defining recession, ABCT and unemployment]