Taking a step back....Can I ask what might be a daft question or two (just for a change)....?
Why were the negotiations between the UK and EU split into 2 mutually exclusive parts in the first place? Isn't that the root cause of the problem here? i.e. the backstop is an attempt to bridge the gap between a withdrawal agreement and a future trading agreement. Wouldn't it have been much more logical to have crafted both agreements in parallel and concluded them both at the same time, thus removing the need to bridge the troublesome gap between the two with this backstop insurance thingamajig?
I'm guessing the answer is something along the lines of: "there isn't enough time between the triggering of Article 50 and the deadline (2 years?) built into the Article wording to conclude both negotiations".... but....if that is the answer, doesn't it suggest that Article 50 was poorly written in the first place? Logically, surely it would be better for Article 50 to give sufficient time for both a withdrawal agreement and future trading agreement to be drawn up together, rather than setting what seems like an arbitrary timeframe to scupper this happening?
Dunno.... Just me thinking out loud, as per usual, and not really knowing much about the mechanics and logistics of the whole Article 50 malarkey....