//NJ’s understanding of things such as import tariffs (to protect local production) seems limited.//
No it isn’t. I understand the EU’s protectionist rackets (sorry, “procedures”) perfectly. But it isn’t the UK’s responsibility to protect the EU’s rackets from incursion.
//Perhaps NJ can explain (under his proposal), how the EU is to preserve widget import taxes in the above scenario (or for that matter, the importation of goods that did not meet EU minimum safety standards).//
No NJ can’t. I’ve absolutely no idea how they can achieve that. But not only is it not my responsibility to find a solution. It is not the responsibility of the UK government to do so either. It is the EU that wants its market protected, so it is for the EU to come up with a solution. Since nobody has any intention of imposing a border in Ireland and since to impose one in the Irish Sea interferes in the internal affairs of the UK (which is contrary to the UN’s founding charter) I suggest they have a problem.
Unfortunately Mr Sunak seems quite prepared to see a part of the UK treated differently to the rest of it and also subject it to the jurisdiction of a foreign court in order to solve the EU’s problem for them. Mr Sunak would make a half decent carpet salesman, but that, I would suggest, is about the limit of his capabilities.