Staff have allowed me to post this topic about cells.
It is possible for a variety of reasons for more than one pokestop or gym to exist in a cell, but it is not possible to submit one and have it go live in an L17 S2 cell that is already occupied.
You cannot submit specifically to be a power spot, but if it is not eligible to be a pokestop or gym because of the L17 S2 cell being occupied, AND if it is 22m from an existing pokestop or gym, they may be added to the pool of wayspots to potentially be used as power spots. It is harder for the community to figure out all the factors that go into the process of specifically which ones are chosen, but it seems that updates to the power spot pool happen around the 4th of each month if they are going to happen.
Everything in this comment is community observation. Niantic does not talk about S2 cells or distance algorithms.