But there's currently no mandate to remain.
You are 100% right. indeed, there is currently no mandate to remain.
There is also absolutely no mandate for no deal. Nobody campaigned on that, and when the outcome was suggested, all brexit supporters (note i dont say voters) laughed it off and denied any likelyhood.
And according to many, beexiteers in particular, there is no mandate for May's deal either (still the only real deal on the table), as it isnt brexity enough for them. I myself would have been perfectly fine with leaving on that deal btw. Lets not forget, that was blocked by the brexiters.
So with no realistic likelyhood of actually leaving, and no mandate for anything, the only way i can see out is a second referendum, with clarity on the actual implications. on the one side, the best deal we can possibly get, and on the other, remain as the status quo and still the less disruptive/damaging outcome.
And if the result is still leave, then so be it.
I honestly do not see a second referendum as some chance to overturn the referendum, i personally doubt the result would. You'd like to think people would at least have the chance (at least those that didn't the first time) to look back at it objectively and reconsider/consider properly.
But i do think it seems the only way through this blockage of everyone (particularly brexiters, as remainers at least are consistent) picking the implied perceptions they like on the day to argue for what they want out of this.
Asking again, after all that has come to light, is not undemocratic. Forcing a no deal is.