The UK has a mandate from the people to leave the EU. Attempting to block the brexit over technicalities in Unionist agreements is a weak position for any politician to take. If the SNP believes they can successfully build a new mandate from the Scottish people to leave the UK, that would be a position of strength and they should work with that.
The question no one seems to have thought to answer until now, is whether Boris or any other politician has a mandate to dissolve the UK in order to do so. Does anyone believe that if the vote had been "Should England leave the UK and the EU" it would have won majority?