That would require many landlords to cut the rent, reducing profit; it's not uncommon for people to be paying half or more of their income on the rent.
Not in my experience. The requirement of 6 weeks' rent as a deposit is, however. I wouldn't be surprised if some ask for 8. This is partly what makes it so difficult to get in to the rental market for some people.
It varies. I only had to pay a month up front, but as others said it depends on the landlord and situation. I believe a month is most common in the UK.