I would agree with @Conifold that the 'gerrymandering' confuses the question. It is an excellent question but could do with a lot of tidying up. The main problem is the casual use of words like 'exist' and 'identity'.
Conifold mentions Plotinus and the 'One' and this takes us to the heart of the matter. If Plotinus had argued that the One exists then he would have fallen foul of the OP's objections. He does not do this. His One transcends the exist/not-exist distinction. It would be incoherent to argue that the source of existence exists. Plotinus' philosophy is non-dualism, not monism. His 'One' is not a numerical quantity.
These complications entail that to discuss these issues at this depth and avoid confusion requires a very clear definition of words like 'existence'.
As to 'identity' and 'definition' again some clarity would help. The question states that a thing with no identity is not a thing. True enough, but there are subtleties. Plotinus argues that things are not really things but mere appearances and all would share a fundamental identity. This 'fundamental identity' cannot be positively-defined or distinguished and cannot be said to exist or not-exist. This would not be a thing but would be all that is truly Real. To exist would be to be created and thus to be reducible in physics and metaphysics.
This is not an attempt to explain this philosophy but to indicate that the subtleties we are dealing with here demand very clear definitions of our terms and concepts, and great care to avoid building unnecessary assumptions into our questions. It may suggest that the way to make progress on metaphysical questions is to keep trying to clarify them. By the time we've defined our terms we've done most of the work.
I believe it is logically impossible for just one thing or substance to exist or for existence to arise from a composite 'phenomenon' or substance. I conclude that Plotinus endorses the only view of these matters that survives analysis.
Great question and I'd agree with your conclusion if not all of your argument.