I see your point about how the border with Northern Ireland and Scotland differ: the former have a formal treaty with the UK stipulating no border shall ever exist, and the latter doesn't. If Northern Ireland leaves the EU, without a deal, the EU are mandating there shall be a border, which is in breach of the treaty. Note that the breech won't be from the UK, if the UK don't enforce any border controls from their side: whether there will be a hard border or not, will be down to the Republic of Ireland who are controlled by the EU, not the UK. Other solutions have been proposed, which don't involve a hard border or the whole of Ireland remaining or leaving the customs union, yet they've been rejected by the EU.
I can see the other side: the EU told the UK there would be a Northern Ireland border, if they leave without a deal, so don't do it.
In spite of the tone of my previous remarks. I don't think it's as simple as blaming the EU or UK for the this conflict. The EU as a whole, including the UK, drafted Article 50 and didn't consider the implications of any state leaving, because it was never seriously considered that it would happen. Blaming either party is counterproductive.
Yes the EU seem united from the outside, but it's questionable how deep it really goes, considering how there are fractions within each country, let alone the entire bloc. Having a pool of smaller nations does make them stronger, but it's also a disadvantage as implementing any change requires them to all agree and compromise. I'm pretty sure opinions range from expelling the UK, with no deal, to giving them an easier deal to protect jobs. As mentioned before, there are many other counties with similar levels of wealth and proseity as the UK, who aren't in the EU and haven't been eaten by the US, so that's nonsense and is part of project fear.
I agree the UK is more divided than Europe, but I think it's the main reason for their weakness, rather than size. The problem is no one agreed with what Brexit should consist of, because no one believed the result of the referendum would be to leave.