2+4. I try not to interrupt students speaking, sometimes (making a basic mistake, e.g. 3rd person present simple I prononce "s", or I show "s" on the board..) I make a gesture, a face, so students can correct themselves.
After written test, essay, composition we write commen mistakes, we correct, try to learn correct forms..., to avoid the mistakes next time.
After a week or two I make an exercise Find mistakes, I use the mistakes from students test or essays.
I like using exercises -Find a mistake - one sentence means one mistake, later not every sentence has a mistake (sometime I tell my students how many sentences are not correct, when the class is good or later at the end of semester I do not say how many sentences are incorrect, how many are correct.
1.Students will make mistakes, it is necessary to tell the students that everybody who learns a language makes mistakes so it is not necessary to be afraid of speaking and making mistakes. Students should talk and talk, should be able to express their adeas - it is our aim. To have a nice, relaxed atmosphere in the class, not to be worried about mistakes. And to have feedback, to learn from our own mistakes and from other students mistakes.
Nobody is perfect.