Steff, others have offered helpful ideas to explore. Here are my thoughts on the specific questions you ask:
180° is the target, but does not have to be exact - 178° to 183° should not make any difference.
20 minutes is again only a target; 25 minutes will not hurt. In fact, the procedure I follow calls for 30 minutes.
With both of these, my understanding is that we are allowing the whey proteins to "denature" so that they can be incorporated into the structure of the yogurt. It is essentially like making ricotta cheese, except that not adding acid or starting with acidic whey - thus the proteins don't curdle, but they do become available and help to make the yogurt thicker.