Needing to maximize shareholder value is a myth. There is no law that requires you to do that - people like to use the idea as an excuse to do scummy business.
Depends. In Michigan it is binding precedent, see Dodge v. Ford (1919).
Delaware corporations must act in the interests of shareholders.
Sure, it's a dubious legal requirement at best. But you try telling people that on an earnings call and watch your valuation plummet because you took a long position and the market wanted a next quarter position. And even if you don't care about selling your stock personally, it does impact your ability to raise funds.