If schools pre-screen for secondaries and they decide not to send you a secondary, will they send you an official rejection email? Same goes for after submitting secondaries: will they officially email you telling you that they decided not to give you an interview?
After the secondary, yeah, schools are all over the place. Some even ghost candidates after an interview! Just stay tuned to the school specific forums. That will give you a good feel for how you are doing as compared to others -- i.e., if a ton of people are reporting IIs and you haven't received one, that's not good.
It isn't over until it's over, and who knows what timing is going to look like this year, but IIs can go out anytime between when secondaries are received and next spring. It is, however, true that some schools will not reject you until the cycle is over, and some not even then!
Before the secondary, not so much. As posted multiple times in multiple places, although people seem to get pretty excited when they receive secondaries, schools love the revenue they generate, so they are pretty liberal in sending them out. If you don't receive one for some reason, you can always try to call the school and find out why, but that's your answer and you might or might not receive formal rejection beyond that.
Most schools that screen for secondaries do it just by using a fairly low bar for GPA and MCAT, although at least one school also seemed to use reported IAs. Of course, there is nothing to stop them from having someone read your primary first, but most don't do that before cashing your check and receiving your secondary.