What I was hoping to suggest was that "Everything" (Sabba) in SN 12.15 was not intended by the Buddha to refer to the totality of the contents of this cosmos. I read "Sabba" as having a more limited meaning, ie the khandhas. I think you will find Ven Thanissaro offering a similar explanation for this term here -http://www.accesstoinsight.org/tipitaka ... .than.html
at footnote #1.
If my reading is correct, it would perhaps be more meaningful to apply the admonition in SN 12.15 to simply be a rejection of the extremes of "The Aggregates exist" and "The Aggregates do not exist".
I think the Buddha was not too concerned with the other bits of the universe that do not constitute Dukkha. The 1st Noble Truth identifies Dukkha with the 5 Clinging Aggregates, while the Culavedalla Sutta elucidates that the 5 Clinging Aggregates are called sakkayo/identity by the Buddha. All self-views are ultimately traced to some form of identification with the khandhas and I suspect that is why the Mulapariyaya Sutta lays bare those roots of self-view in their various manifestations.
If a unicorn is therefore not part of the Sabba menagerie, should we apply SN 12.15 to take a middle stance regarding its existence or non-existence?