To answer your questions directly:
Any corporation, religious or otherwise, has bylaws and charters that call for directors and leaders to be voted in my members with a voting share. There are many kinds of constituencies and allowances that can be unique to any corporation. (Even if you started a small business as an "S" type corporation, and you are the only owner, you will still have to certify that as they only voting member, you voted yourself control over your corporation yearly, or as often as your charter allows for. In almost all cases in the United States, this is done yearly...)
These bylaws must meet the standards set by the United States Government.
A religious corporation can select and set the standards for voting members.
The charter of the WTBTS provides for a "Board of Directors"
These directors must be voted in yearly. This is also in the charter of the WTBTS
In the 1920's,1940's, 1970's, and 1990's the charter and bylaws have been amended to allow for new "understandings" of the Governing Body. The latest one allowed for non anointed ones to serve as corporation members. That is why Don Adams became President of the WTBTS after Henschel died.
Because religious corporations in the United States are used for business purposes of the leadership of these religions, it is now generally understood in American law that the directors of these corporations do not supersede the "ecclesiastical" leadership of any religion. (i.e. The Governing Body, or Archdiocese, etc etc etc, oversees any and all business corporations set up by said religious leadership, including the President(s) of these corps) This also allows the Ecclesiastical leadership of Jehovah's Witnesses (The Governing Body) to select members of the WTBTS, who will be given a voting share. Obviously, the most trusted and loyal are selected.
Once the Governing Body understood this, the newer members that is, then they gave up the business work, and allowed the Great Crowd to be in the corporation. I hate to tell you this, but all that buzz over the "given ones" of the great crowd being helpers to the Governing Body in the early 90's was primarily to pave the way for them to be the officers of these business corporations. They report directly to the Governing Body.
Why then, all the ceremony for the once a year meeting? It traces directly to the coup of J F Rutherford in 1917. Fearful that someone else would or could do precisely as he did, Rutherford arranged to make sure that he would be voted in every year until his death. (remember, what Rutherford did went directly against C T Russell's last will and testament) Rutherford used the bylaws of the corporation back then to take the presidency away from the editorial board Russell had set up, then changed those bylaws later to consolidate his power.
In the 1940's, after Rutherford's death, Knorr centralized and camouflaged the role of the President of the WTBTS, and some changes were made to the charter, again, to consolidate power. The board of directors was a bit more emphasized starting with Knorr, and esp grew in the 70's when the Governing Body was given some teeth in 1976.
Ok, this is becoming another JW history lesson. Sorry. I could go on, but I am off to look for my missing life... Hope that clarified what I said....