The reason I asked about identifying the submitter is that if the same person using multiple accounts also submitted the PokéStop, I thought it could be relevant when reporting potential abuse.
Also, just to clarify, when you say it would be difficult for Niantic to take action, do you mean that reports based only on suspected multi-account use are unlikely to lead to action, unless there is clearer evidence (for example, location spoofing or other violations)?
I do not speak for Niantic, but I know they want conclusive evidence before they take action on accounts, based on comments that staff have made here. If you think that abuse is happening, they will investigate.