In fact, I disagree with barely anything you've said, though I might have different emphases. I particularly endorse your penultimate paragraph, and your point about the way the 2017 result has been spun by Tory politicians as an endorsement of Brexit, when in fact it was barely as issue.

There is of course an irony in the fact that, had the referendum been mandatory, the irregularities in its conduct would have resulted in its being struck down by the courts, but because it was merely advisory it has assumed the entrenched position you refer to. And you're right, my objection to the Lib Dem position is primarily political rather than legal; but politics is such that relying on the legal makes you appear legalistic, and logic is read as sophistry. In this case, hypocrisy is added to the mix: this policy would be a problem for any party, but for the Lib Dems, who have made so much of the inadequacy of the FPTP system, and indeed sought and obtained a referendum of their own in an attempt to change it (irony on irony!), it looks worst of all.

I think the Labour position is actually pretty reasonable - however, it will be impossible to sell, and I'm not optimistic about its doorstep reception.