Good questions and I wont pretend to have all the answers, but have some suggestions.
Shares that are being short sold are borrowed usually from institutions. The instos are paid "interest" on these shares (its not interest, but the concept is the same). So the insto has the stock, gets the dividends etc. and earns extra "interest.
The shares are borrowed without a necessarily fixed time frame for return. They can be called back by the owner of the shares at his or her discretion.