Technically speaking, anyone can open a 529 account for any US citizen, and an individual can have an unlimited number of accounts in his or her name. So one does not need the parents to open an account.
True. I set up 529s for my nephews by myself.
withdraws will be counted as taxable income for the student.
I am pretty sure this is not correct. The withdrawal for the student can be counted as an expected contribution towards the next year's tuition, though, which is why it's good to make the 529 distribution for the student in her last year of college.
You can gift (IIRC) up to $15,000/year to an individual without that individual having to pay taxes on that money.
This is not correct. You can gift $15,000/year to an individual without the DONOR having to report that gift and count the excess over $15,000 against his lifetime federal transfer tax exemption amount. But with the lifetime exemption currently at $11.4 million, the transfer tax affects very few people nowadays.
Also, there are no income taxes on gifts, and the donee is (almost) never going to have any transfer tax liability on a gift, either. This has nothing to do with the question posed by OP, but I wanted to clarify that for anyone else reading this thread.
OP: Since you don't want to require the money to go to education, the simplest method is to keep a separate account in your own name and give it to your niece when she is of an age that you believe appropriate. Another method is to set up an irrevocable trust for her benefit and put the money in an account in the name of the trust. As long as she has no rights to any of the trust principal or income (a fully discretionary trust, with the trustee having absolute control), the money should not be counted towards her expected contribution to schooling. The trust terms can allow her demand rights at a later age or upon graduation from college (so she could pay off any loans if she actually graduates). There are all sorts of caveats that you can include in a trust like this, but it's, I admit, not ideal. And you can't make her parents the trustees, either. This is, admittedly, more complex, but it ensures that she will get this money. Or you could name her as a beneficiary under your own estate plan, just in case you don't survive until she reaches majority.