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'. 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. Rather, his One transcends the exist/not-exist distinction. It would be incoherent to argue that the source of existence exists. Plotinus does not do this. His philosophy is non-dualism, not monism. 

These complications mean that to discuss these issues at this depth and avoid confusion would require 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. Plotinus argues that things are not really things but appearances, and all would share a fundamental identity. This 'fundamental identity' cannot be positively- defined or distinguished (from what?) and cannot be said to exist or not-exist. Yet this would all that is truly Real. The word 'Real' would not mean the same as 'exist'. 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 just to indicate that the subtleties we are dealing with here demand very clear questions and definitions of terms and concepts. 

All this may suggest that the way to make progress on metaphysical questions is to keep trying to clarify them. It is not easy to do.  

I believe it is logically impossible for just one thing or substance to exist or for existence to arise from a composite phenomena. This suggests 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.