Ah, the 30-minute mozzarella -- not what your recipe calls it, but how it is often known around here. I love how it looks so easy on the website, TV show, etc. ... but an awful lot of folks here on the forum find mozzarella to be one of the most difficult cheeses to get consistent results.
As the recipe you have linked to says, the milk makes a difference for sure. But I noticed, in their pictures, that even with the good milk, that presumably produced the good final results that they show, this acid-added approach generates a somewhat crumbly looking curd. That has always been my experience with this approach -- adding the acid tends to curdle the milk, and then when the rennet it added you get a curdled curd, if that makes sense.
You can also make mozzarella a more traditional way, by adding the appropriate culture(s) and letting the bacterial action generate the required acid. Generally with this approach, you will add the rennet while the milk is only mildly acidic, let it gel, cut the curds, maybe cook a bit, and drain them -- and then wait for the curds to continue to acidify until it reaches just the right level of pH, around 5.3 if I recall correctly. If the pH is too high, it won't stretch; if it is too low, it will crumble up.
I have always wondered how well the "30-minute" acid-added approach does in generating exactly the right pH. With the traditional method, you can start testing the curd to see when it gets to the magic stretchable point; the pH starts higher and drops over time under the action of the bacteria. But with the acid-added approach, it is kinda all or nothing -- either you have the right pH, or you have ricotta. Or tough cottage cheese.
I realize that none of the above is directly addressed to your questions, but maybe it helps give more insight into the many issues that can afflict mozzarella!
