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? 

Breaking a promise made to Allah

Q:

1. If I make a promise and say insha Allah after it, would I have to pay kaffarah if I break that promise? For example, if I say I promise not to eat a chocolate for three days insha Allah and end up eating the chocolate would I have to pay kaffarah for breaking that promise?

2. If I have to pay kaffarah, how much do I have to pay?

3. If I have to pay the kaffarah and I made a few other promises just like that and they're all broken, do I have to pay separately for each, or once for all of them?

Comprehensive tajweed kitaab

Q: Several years ago, my Qaari Saheb taught me proper, comprehensive tajweed principles along with their application. Eventually, I forgot the principles although if I am asked to recite something, I can Alhamdulillah recite it properly.

I wish to revise tajweed. Please suggest a book (in any language: English, Urdu, or Arabic) that explains the principles comprehensively.

Repeating salaah if one found out that wudhu was not valid

Q: I was sitting after performing salaah when I noticed that there is a black, sticky substance on my foot through which water could not pass. It was the size of a whole round fingernail, and I had not seen it before.

Then, my brother told me that he saw it since 3 or 4 days on my foot. I had also led jamaat during these days. Will I have to repeat all the salaah that I had offered on these days? Will those who followed me in jamaat also have to repeat their salaah?

Husband says to wife, “If you ever again say to me ‘this is your fault/this is my fault’, then Allah’s qasam I will switch off my phone and will never open it again.”

Q: Husband says to wife, “If you ever again say to me ‘this is your fault/this is my fault’, then Allah’s qasam I will switch off my phone and will never open it again.” If wife ever utters those statements in future and the oath breaks, will the husband have to pay kaffarah or what will he have to do? Since it is not possible to switch off the mobile forever.