Authentic Brands Group had bought the intellectual property of Muhammad Ali. This includes his 5000 photographs and pieces of memorabilia and the right to market his name and image. ATG holds similar licensing rights for Elvis Presley and Marilyn Monroe. The terms weren't revealed, but it's estimated Ali could be marketed for around $40,000,000 a year if done properly. http://espn.go.com/boxing/story/_/id/9999376/abg-buys-muhammad-ali-estate-licensing-rights
You can't be the estate of someone who's still alive, because it doesn't exist yet. By definition someone has to die before an estate can be established.
Not true. That is one legal definition of estate, but not the only one: http://definitions.uslegal.com/e/estate-law
It isn't his estate as such, it's the marketing rights to his name, which Ali himself sold some time ago.