Mistakes in Salah

Question

26th December 2020. Assalaamu 'alaykum wa rahmatulllaahi wa barakaatuhu respected ulamas. This is an EMERGENCY: I am writing to you about "DISTORTION OF SALAAH AND QURANIC VERSES IN SALAAT IN JAMAAT". Please, FOR THE SAKE OF ALLAH, kindly answer as CLEARLY as possible to ALL FOUR questions below. The imam in xxxxxxxxxxxxxxx at xxxxxxxxxxxxx Road in xxxxxx does the following during salaah in jamaat: - Most of the time (more than 75% of the time), he says Allaahu akabaran instead of Allaahu akbar when starting salaah and in the middle of salaah (before changing postures). - While performing tilawah during salaah, he says "ran" at the end of verses where there is " letter raa" with no fatha nor tanween on the "raa" letter, for example: a) He ends ALL verses in surah Al Qadr with "ran" b) He ends the first few verses of surah Al Muddathir with "ran"and so on. Question no. 1: Is it FORBIDDEN to read salaah behind him? Question no. 2: In the case that it is forbidden to read salaah behind this imam, will it be a sin to perform salaah at home instead of performing salaah in jamaat? Question no. 3: What about the salaat we have already read behind him: are they VALID OR NOT? Question no. 4: What will we answer to Allah sub-haanahu wa ta'aalaa on the day of qiyaamah about this situation and our salaat? Please kindly help and may Allah sub-haanahu wa ta'aalaa reward you in both worlds. Thanking you beforehand. Jazaakallaahu khayra. Was salaam.