I'll go with it depends as well - I have worked in music industry with a rock band before (although I must add this wasn't in USA and I do not know about specifics on entertainment laws). In my personal experience I haven't seen artists giving out power of attorney and I don't think it is common practice to give such full control.
This is how generally things work
an artist will have a manager (sometimes there will be a general manager and there will be another separate manager for tours - if the artist is really famous and has big tours), a publicist, a group of different lawyers/ legal firms ( 1 entertainment lawyer/firm for deals etc, 1 business lawyer/firm if they have their own production/record company etc and 1 general personal lawyer/firm if needed), an accountant/accounting firm, a representative assigned to them from the record company and a bunch of assistants working for either the artist or the people mentioned above. ( I was an assistant for the band).
record company representatives I dealed with kinda had a temporary position in arranging initial promotion of the newly released albums. the rock band I was working was signed by universal music and their contract had a clause saying that they would promote the albums they released by doing a number of signings, TV shows, appearances etc. This was all arranged by the record company. Later similar tasks (appearances, interviews etc) are done by the publicist / pr person/company.
concert etc deals were done like this: generally the manager would ask the artist what they want to do and the artist would give basic guidelines such as they would like to tour for 3 months and take a break, number of shows etc etc. than the manager arranges everything , which generally involves a lot of back and forth discussion with the manager and the organizers such as organizers ask for 20 shows, the manager replies back with 15, organizers want you to do meet and greet with fans, manager replies with conditions etc etc. It generally takes a lot of discussion. once everything settled the organizer's lawyers draft the contract which is sent to the lawyers of the artist and then it goes back and forth as well for several times as generally they would disagree on wording, organizers would want to protect themselves the artist's lawyers would want to protect the artist. Now artists generally have no idea about this discussions, modifications, this back and forth communication between their manager, lawyers and the organizers or about the details of the concert deal etc.
Once everything settled the manager and/or lawyer will bring the contract to the artist to sign. the artist will only get the summary of the contract such as " we are going to do 40 shows in 3 months starting from April and will be paid this much". Then the artist signs the contract. Once again I must add I have never seen artists allowing their managers, lawyers etc to sign for such stuff.
And seeing that Michael indeed himself signed a lot of documents (the AEG contract, firing of Tohme, Rowe etc etc). I personally don't think that he gave signature power to anybody.
Then a lot of work is done by the assistants. We used to get a monthly planner - in short telling us what are we doing what day and then we'll get a weekly call sheet type of thing which lists your activities in detail - such as the car service will come to pick you up at 5 am, we'll be at airport at 6, the plane leaves 7, we'll do an interview at 1 pm. the sound check is at 3, we'll arrive at venue at 7pm etc etc. Needless to say all of these things are arranged by the assistants and artists just follow the outline that they are given.
Now a lot of times the contracts are cancelled too, which is very common. Basically sometimes artists change their minds or something comes up etc. In this cases once again managers and lawyers step in to come to a middle ground such as agreeing to do something in a future date, agree to do something else, pay a penalty or even go to court.
wow this was one long post and hopefully it wasn't too boring to read
edit: when I applied for that job I was initially interviewed by the manager and then I was interviewed by the band (they were a 5 member group and only one of them were doing the interviews) and the band (that one member) decided to hire me and all members had signed my contract at that time.
I also tend to think that high level hirings are generally done by the artists themselves. such as the artists will hire the tour manager but allow him to hire the tour crew.
