1.) How long can pizza dough stay in the fridge before it becomes unsuitable for pizza? I'm talking about already risen dough - balls that are ready to be stretched and baked.
Since you said specifically "already risen dough - balls that are ready to be stretched and baked," I think an appropriate answer is: Not long, usually.
If dough balls are ready to be stretched and baked, they will soon be beyond ready to be stretched and baked. One of the reasons why you may refrigerate dough is to keep it not quite ready to be used
for a reasonably long time (like a day or two). With dough that's not quite ready to be used, you make it ready to be used by removing it from the refrigerator for an hour or several hours before baking. If you do it right, "not quite ready to be used" may span a couple days or longer.
You probably don't want "already risen dough" in the fridge, in most cases.
There are so many different possible answers to your question, which depend on many different variables. Like what style of pizza are you talking about? If you're talking about NY style, I find it easy to make dough that's usable after as few as 24 hours or as many as 72 hours but best after 48 hours. With stiffer doughs, such as my interpretation of Giordano's style, or cracker style dough, the window of opportunity is probably much longer.
The sooner a dough is ready to be used, the sooner it will be unfit to use. If you make dough that's ready to be used in an hour (which probably wouldn't be refrigerated), it'll likely be unfit to be used within another couple hours (or less). Such a dough would contain a ton of yeast. Conversely, you could probably make dough with a weeklong window of usability, but it would probably first take a week for that dough to be ready. This dough would contain very little yeast.