Jamaat Salaah and Imaamat

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? 

Covid positive person attending the musjid

Q:

1. If one is diagnosed with Covid, should he isolate and read his salaah at home, or should he go to the masjid if he is affected mildly (i.e. body strength, very light cough, and fever, minor difficulty in breathing, etc.)?

2. If a person who is Covid positive and goes to the masjid and the other musallis are aware that he is positive, will he then be sinful for inconveniencing Muslim brothers as unfortunately not everyone is comfortable being around the sick these days?

Filling the gap in the saff if the muqtadi leaves during salaah

Q: There is a rule in Bahishti Zewar that it is makrooh to stand alone in a row. The person should pull a person back from the row before him. What happens to the gap formed by the exiting of the said muqtadi to fill the previous row?

If a Muqtadis wudhu breaks during prayer, should he just leave and leave a gap in the saff? What should he do in this regard?