Stating that Varadkar did not threaten violence is not a semantic point. Claiming that he did his hyperbole.
I am not a SF supporter. But if they were being deeply cynical in pursuit of their goal of a united Ireland they would probably just sit back and say "bring this border on". That they do not so is (I'm guessing) probably down to some combination of real abhorrence of a possible return to violence and concern that any such border would undermine their credibility to the advantage of what are loosely described as dissident republican groups.
I am not aware of anyone having recourse to an appeal to the UN Charter of Human Rights. Hyperbole again? But the reference to custom checks seems to suggest that this relates to a trade issue between two separate countries. Even as such, this would be highly disruptive for people along the border where cross border trade is simply integrated into every day life.
But it is much more than a trade issue. The NI state is largely comprised of people with two different identities - British and Irish (acknowledging but omitting the nuances within these, and omitting the smaller third group who identify as neither). The GFR agreement was negotiated in the context of EU membership. Arguably its greatest architect, John Hume, always emphased the EU dimension. Constitutionally the GFR maintained the constitutional status of NI within the UK as valued by those with a British identity, but the invisible border is of of a symbolic counterweight to this for those of a nationalist identity (along with other provisions of the agreement around power-sharing, equality and the prospect of a border poll). The re-installation of a border will be seen as a provocative and hostile act by at least a section of the nationalist population - a not insignificant section.
I think it was a miscalculation for Varadkar to speak about a united Ireland recently - even if he was not advocating for it. I don't think it was of any value - even from an electioneering point of view.
But once the Brexit issue is settled one way or another the Irish government will need to go about establishing a forum on the future of Ireland. A border poll is inevitable within the next 10 years. Unlike the Brexit referendum in the UK, there should be at some sort of clear understanding beforehand of what people might be voting for - what would a united Ireland look like constitutionally and administratively and what would be the stages in its establishment. What the outcome of sich a referendum might be I don't know. But how Brexit pans out will be a factor.