Just some quick thoughts. If there is a hard Brexit without a backstop then the UK will have breached the Good Friday Agreement. The GFA will then be dead. It is the GFA that guarantees the north will not be handed over to the republic without a majority of voters approving the decision in a border poll. So without the GFA a future UK PM (e.g. noted Irish sympathiser Jeremy Corbyn) could just hand over the north. Is that correct?
Personally I think that would be a very bad way to go about doing things and would only lead to trouble, but if it is the case perhaps it would be a point worth making to the DUP when the are making their demands on how the border is handled.