How can I substitute dutch chocolate cocoa for flour and maintain proper recipe PH in this cookie recipe?
I doubt this is an acidity issue. Most flour is pretty neutral, the reason your cookies seem dry is that they are dryer. Cocoa powder is about 7% moisture, source doc here, whereas flour is about 14% moisture, see this site for details. Add to that cocoa powder is very fine, and likely more absorbent than the flour, and the fact that the dutching process makes the powder more absorbent. So the cocoa you add is dryer, and absorbs more moisture from the rest of the ingredients and you will need to add some in to replace it. 

Looking at the math .75 cups of flour is about 85 grams, 14% of 85g is 11.9, rounded up is 12g. Cocoa has about half the moisture, so you have lost about 6g of moisture in your recipe, which is just over a teaspoon, under ideal conditions. However, often when flour is stored it absorbs more moisture from the air as it is not kept airtight, so you could be losing more. A dry cookie dough will sometimes have trouble coming together, and be crumbly, which is something I would look for in your case. I would try adding a teaspoon of milk or water to your recipe and see if that creates the right consistency, and then add another if it still wants more.