1. The backstop id just in case of a Brexit with a deal, since this includes a 22 month transition period. The Backstop id for the transition period, not for the Brexit itself. A no-Deal Brexit nullifies the need of the Backstop.
2. True. It is also explicitly her duty to call for such a poll when the people wants to become part of Ireland. In other words, it's time to plan if she wants to act on her duty. On a somewhat related and sad side, it looks like Brexit could refuel the Troubles even without any hard border: https://www.gov.uk/government/speeches/security-situation-in-northern-ireland
3. Deal or no deal, that doesn't change anything here. In fact, the UK already implemented them and several countries already protested against them (New Zealand and Taiwan at least for my knowledge, but there might very well be more)
4. I literally facepalmed when I read that. That's just border controls, not a fucking Berlin Wall. Besides, if a physical barrier inevitably leads to militarization, how come the European borders were not controlled by military force for decades before the Schengen agreement? Or that there's no military presence at the EU borders? Maybe because in inevitably leading to militarization is complete bullshit? Pretty much! Ireland can with no problems at all choose both the Customs Union and uphold the GFA. There's a clear difference between not wanting to do something and not being able to do so.
5. Leo is believing that an agreement can still be found, thus clinging on the backstop agreement. But I think we both agree here that there will be no deal Brexit. But I agree the Taoiseach is getting increasingly desperate as he tinks the backstop is the only solution... hence why he urges the UK time and again to accept the 585 page-long deal, as without a deal, no backstop either. One must note thought that until two weeks ago, May also urged the parliament in favor of the backstop, the fact that she's not doing that anymore just shows that the parliament isn't willing for it and the EU won't renegotiate the deal. We'll see in the coming days how Leo's position will evolve I guess.
6. Look at number 2
And if you don't know by now that you're in total denial, not everything collapsing around you could point you in the right direction
1. The backstop and the whole of UK remaining in a customs union is indefinite so how do you propose to solve this without a no deal exit ? The Labour party is fucking themselves either over a people's vote or a permanent customs union. The Conservative party is fucking themselves either over an indefinite customs union or having no customs union at all. The ERG (along with the DUP) have Theresa May right in their crosshair just like they did with David Cameron ...
2. Neither the DUP or Sinn Fein has any say either way unless they somehow won the majority in the House of Commons then they could in theory appoint whatever Secretary of State they wanted for Northern Ireland. It's ultimately Karen Bradley's choice to call for a border poll to which she recently rejected to Sinn Fein and calling for one doesn't make any sense either since in the last general election the DUP along with Sylvia Hermon won the majority of votes and seats in the parliament representing Northern Ireland so there's no reason to call for a border poll when the people of Northern Ireland want British unionists to represent them. Maybe if the DUP get decimated in the next election and if Jeremy Corbyn becomes the prime minister but right now it's not looking so good for Corbyn's chances ... (even if Sinn Fein held a majority in Northern Ireland a Conservative government can still reject calls for a border poll so only the prime minister has the power to affect whether a vote will be held or not)
4. And what if Leo keeps refusing ? Ireland NEEDS something like a Berlin Wall because Irish nationalists in Northern Ireland will get very angry over being separated and that poses border security risks to personnel. Ireland and the UK has tried just doing customs checks before but lo and behold both of them were finding themselves fighting against paramilitaries. The extreme nationalist will probably try using violent methods to breach customs checks and they'd see it as a betrayal of sorts from the Irish government. Can you be absolutely certain that those extreme Irish nationalists won't revolt ? The EU has made it clear that it can not ignore the risk of a backdoor anymore and right now Irish goods entering the whole EU continent aren't checked because Ireland makes sure that shipments arriving from countries outside the EU are compliant with EU customs standards along with the UK since both have no need for a physical border in that case but a no deal Brexit derails this arrangement since the UK aren't obligated to see eye to eye with EU customs standards ...
5. Leo is not wrong in thinking that the backstop is the only way for them to realistically keep the GFA and be in the single market but the reality is that the UK isn't going to draw the line so someone else needs to whether that's going to be the EU or Ireland. The only other option left to be able to keep both is if Ireland accepts a China style "one country, two systems" arrangement whereby they have extended areas that includes the Irish border under a different "designated special customs area" which would essentially mean the Republic of Ireland would now have internal borders just like China does with Hong Kong but this would split the Irish communities and we don't know how Irish nationalists arriving from Northern Ireland would react to this ... (if Ireland wants a border then it should do that to itself to meet two different customs instead of expecting the UK to accept a backstop)
One way or another the UK will be taking something as a collateral in the scenario of no deal ...
1. The backstop is a part of the divorce deal. Since UK is heading straight into a no-deal Brexit, this part also falls through. In other words, with a No-Deal Brexit, the Backstop also automatically drops off the table and thus the UK is not part of the Customs Union anymore then in any way, and will have to pay import taxes if they want to export to the EU.
2. They have no say, but it's the NI Secretary of State's duty to perform such polls. Also, it's her duty to reflect on people's interests
4. No need for such security. Besides, if the Troubles come back, they happen in Northern Ireland, so it would rather be in UK's interest to raise the wall sky-high instead of Irelands to keep the Nationals from supplying themselves with everything they need. For the rest, look at point number 1
5. Leo is right in a way, as it would keep the spirit of the GFA, but not the letter, which doesn't say anything at all about or against border controls, So these can be put in action without breaking the agreement.
LurkerJ already posted the collateral, with some jobs being lost on the continent as there will be slowing trade with the UK for a while at least. The economy of the EU will get a small hit, but nothing near the hit it will be for the UK.