Istinja

Q: Can tissue paper be used as an alternative of water for istinjaa? That is to clean the specific areas while using toilet. If yes, then is there any specific method to be followed, or just clean until we get natural satisfaction? Please confirm in all of the possible conditions:
a) If  clean water is available.
b) If there is doubt there the water available may not be clean.
c) If no water is available at the spot.

Q: I could not remember if I washed after I went to the toilet when passing urine. What should be done in such a situation? If anything other than ignore it. What happens to my Salaah because I ignored it now and maybe before too?

Q: Please explain how is covering the head a sunnah of going to the toilet.