You can use any state's 529 plan.
There is no federal tax deduction for 529 contributions. Some states make state tax deductions available. Check with your state tax authority.
Other people will be able to contribute to a plan that you set up.
Maybe. In what way depends mostly on who the account owner is. If you own the account and a niece or nephew is a beneficiary, any distribution will count as untaxed income to the beneficiary for federal financial aid purposes. Federal education loans are an entitlement; as long as the student is a U.S. citizen or resident alien, the loans are available. As student and parent income and assets increase, the proportion of loans that are subsidized (interest is paid by the government while in school) decreases.
As far as I know, every state affords the same tax free benefit to 529 qualified withdrawals.