Can you use StackReference without using the pulumi backend? what would I use as the unique stack name?

Yes, you can. However, IIUC, other backends don’t support Pulumi’s notion of organizations and projects and such, so you have manually namespace things like described here: https://martinjt.me/2021/03/04/pulumi-multiple-projects-with-custom-backends/ (this article also contains an example StackReference)