Method of Salaah

Muqtadi saying the takbeer when moving to different postures

Q: Is saying transitional takbeer (Allahu Akbar) when going to ruku, sujood, qiyaam obligatory upon the individual praying behind an imam or can he just follow the imam.

I have performed many prayers behind the imam and I thought I wasn’t obligated to say the takbeer besides takbeer al ihram. Would my previous prayers be invalid otherwise even if I were unaware? 

Method of making salaam with the imaam after salaah

Q: Should a follower in salaah make salaam:

1. Simultaneously with the imaam

2. After the imaam (such that he waits for the imaam to complete his right salaam and thereafter does his own right salaam, then waits for the imaam to complete his left salaam and then does his own left salaam)

3. Do the right salaam after the imaam but the left salaam simultaneously with him

Sometimes an imaam has completed his head movement but is still stretching the words of the salaam. If we are to do our salaam after the imaam's, should we do it as soon as he has completed his head movement or should we wait until he has also completed uttering the words?