So I've got a rather uninformative thread going on in Mechanics about some of your questions.

Here is my understanding, at a certain point, Maybe 9 speed and up, there started to be specific derailleur movements corresponding to specific cable pulls (indexing). Below that threshold, the ratios were the same, above that, you need to match systems. There is some mixing and matching that can be done but don't ask me.

As for friction, it can do anything but may be limited to how precise your hands are.