Good idea. I'm very much against holding referenda on vague, ill-defined proposals which charlatans can twist any which way they please. But a referendum instructing the Scottish government to open independence negotiations with Westminster and membership "exploratory discussions" with the EU doesn't impinge on Westminster prerogatives - to be followed by a formal referendum on the precise terms negotiated.

Of course if Westminster plays "silly buggers" and seeks to impose impossible divorce terms onto Scotland they could inflame the situation to the extent that Scotland proclaims a unilateral declaration of independence and England is left with military force as the only means of maintaining the Union. Then the situation would get very nasty indeed and perhaps provoke US/EU/UN intervention...

Index of Frank's Diaries

Maybe.

It would seem to me to be a similar road like the one they started on in 2014:

Legality of a referendum

The Scottish government insisted in 2010 that they could legislate for a referendum, as it would be an "advisory referendum on extending the powers of the Scottish Parliament",[17] whose result would "have no legal effect on the Union".[16]:17 Lord Wallace, Advocate General for Scotland, said in January 2012 that holding a referendum concerning the constitution would be outside the legislative power of the Scottish Parliament[24][40] and that private individuals could challenge a Scottish Parliament referendum bill.[41]

The two governments signed the Edinburgh Agreement, which allowed for the temporary transfer of legal authority.

As far as I know, the Edinburgh Agreement came before there was any serioud legal challenges.

If I understand correctly, the objection is based on the subject matter - Scotland's relationship in the UK - isn't decided by the Scottish parliament. And any legal proceedings about this kind of political matters are always a political matter even if it is fought in courts.