Various masaail of Jamaat Salaah and Imaamat

Q: Can a musallee reserve a place in the saff for another musallee?

Q: I would like to know that when should we begin salaah with the imaam, after takbeer in namaz or before namaz. Can a person do itteba of imaam when he makes the niyat?

Q: Suppose we are offering 4 rakaaat namaz as a muqtadi and we are in the 2nd rakat of prayer and as we know in the 2nd rakat we are suppose to say only "At-tahiyyaat" but mistakenly you also said Durood Ebrahim, then what should one do if he is offering the namaz as a muqtadi?

Q: If a husband and wife would like to peform jamaat salaah at home, would they stand just like how two people would normally stand?

Q: Can a non Muslim join in on a jamaat for salaah at the end of the rows to gain an experience and does the jamaat count?

Q: Is it permissible to just put my hands in the position next to my ears just before the imaam says takbeer-e-tahreema in preparation or will this be counted as سبقت علي الامام?

Q: If a person is wearing a job ID card with his picture (the card is about 1 inch in size) stands next to us or behind us in Salah, would it affect our Salah?

Q: Our imaam offered 3 rakahs instead of 4 rakahs in asr farz Salah and ended the salah. No one interrupted him for his mistake and all the people ended offering 3 rakahs, then people said it was only 3 rakahs completed so the imam repeated the 4 rakahs again and every body prayed 4 rakahs again.

My question is if a person is behind a imam in the above case and is aware that the imam has made a mistake can he individually continue the same Salah and complete it ?

Q: If I miss saying "​Aameen" when the Imam finishes Surah Faatihah, will it affect my namaz? 

Q: At what stage should the Imaam commence reciting the Takbeer when going into ruku and sajdah? Should he commence as he changes posture or is it permissible for him to recite this Takbeer after going into ruku or sajdah?