I don't think "a" cherry picked customs union will fly for a transition period though. I don't see how that's practical. Why would you go for a CU on certain goods now and be on WTO terms for everything else?
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Compared to the US, Russia, the EU, China and South American alliances, on your own you'll be pretty small. That being said I was mostly kidding.
About the backstop. It is not an emergency button anymore. It is fairly obvious it can't be avoided. Nobody has come up with an alternative to it, and I doubt anybody will.