Tayammum

Q: What are the conditions to make tayammum on a wall? A wall which is painted with oil, is it ok?

Q: If someone is ill and has some disabilities, doing wudu and praying salah will increase the illness. He doesn't have anyone to help him. Can he do tayammum and pray sitting down?

Q: If water is available but a person is sick, will it be permissible for him to make tayammum?

Q: How big must a stone be for us to be able to use it for tayammum? Can it be a small stone that we roll around in our hands, or must it be big enough to cover the entire hand?

Q: If the person has access to water and isn’t ill and he can use it and suppose this person does the actions of tayammum, is it said that he did tayammum but it’s not valid or is it said that does not count as tayammum as water is present?

Q: If one has to perform tayammum, when putting the hands on the earth, does he have to rub them or if he just places them on the ground, is that sufficient?

Q: After janaabat, if I do tayamum instead of gusal due to cold so in this case can I offer my prayer in the morning after ambulation?

Q: For the permissibility of tayammum, how many kilometers is one shar'ee mile? 

Q: Is there khilaal of the beard and fingers in Tayammum?

Q: One of my colleagues told me that you can make tayammum by touching the seat of the car or the floor of the car while traveling if you don't have water for wudu. Is this correct?