food - Kashrut of rennet for cheese & Difference between rennet and gelatin

I actually have 2 questions which are related.

1. Traditionally, making cheese involves using rennet, a substance derived from animal stomach. If the rennet is derived from a non-Kosher animal, it is non-Kosher (making the cheese non-Kosher as well). From my understanding, the reason for this is the rule "כל היוצא מטמא טמא" - all that is derived from an unclean (animal) is also unclean. But, even rennet from a Kosher animal is not considered meat because of the process involved in extracting it. So, why the difference? Why does the rennet lose its "meat properties" but not its "unclean properties"?



2. Suppose we understand the differences between the meat & unclean properties, what is the difference between rennet and gelatin made from non-Kosher animal bones? I read that the opinion of the Shulchan Aruch (99) is that (completely) dry bones of non-Kosher animals are considered Kosher, and so gelatin from those bones would also be considered Kosher. Why does the rule of "כל היוצא מטמא טמא" not apply here?