Taking a salary from a person who took out an interest-bearing loan

Q: I am a student. I teach my aunts son mathematics. This is my first job as a student. I always wanted to earn halaal or I have a wish that my first salary or income will be atleast halaal.

My question is, somehow, I know that my aunts source of income comes from riba or a bank loan or something or fraudness which is haraam. Is my salary halaal or haraam? I am confused. Please explain, if it is not halaal then I will teach him for free. 

Advising a friend who is involved in a haraam relationship

Q: My friends brother is currently in a rehab. She was very close to him and she's finding it very hard. Shes in a haraam relationship due to this. I tried advising her but she said that it keeps her mind of her brother and that she doesn't know how to end the relationship. She feels that someone else will be able to fill the gap her brother left behind. What advice should I give her? 

Leaving out sins because of fear of people

Q: Is committing sin and not having fear of Allah but having fear of people or to follow our desires considered to be shirk? Kindly answer all 3 different scenarios.

1. If we commit sins like listening to music, giving bribes or consuming riba secretly when we donate and have people watch us because we fear for our reputation, will it be considered shirk?

2. If we commit all such sins due to our weakness for money or for our love of listening to music yet we know that it is wrong because shariah does not permit these sins, will it be considered shirk because we are following our desires over following the commands of Allah Ta'ala.

3. If we stop consuming riba because government disallows it not because Allah has forbidden it, will it be considered shirk?

Giving kaffaarah for missed salaahs

Q: I would like to know if there is a way I could pay for the missed salahs in my case.

After giving birth my menses cycle did not go normal and was bleeding continuously due to feeding. An Aalimah told me I needed to refer back to my habit previous of pregnancy which was 23 days of tuhr and 5 days of haidh. In this way while doing the calculations almost 2 years later I have discovered I have missed about 400 salah.

With 2 children it may become difficult to perform qadha. Is there a way I could pay for this miscalculation or does Qadha have to be performed?