Why pre-mix baking soda into 2 tsp of milk?
Mixing it with milk (or liquid in general) is probably just to remove clumps. It clumps easily, especially when things aren't perfectly airtight (probably more common in your grandmother's time), and mixing in a small amount of liquid is an easy way to make sure it all breaks up.

I'm less sure about the alternating. It is pretty common to alternate wet and dry as a way of getting things evenly mixed and avoiding lumps, so it's possible it's a variation on that. The paste might be too thick to easily mix into the wet, but also wet enough that it'd tend to stay as a lump if you mix it straight into the dry, so alternating wet-dry-paste might get you more even mixing? But if you're able to mix the paste smoothly into the wet, it's completely fine to just do that.