Putting Northern Ireland outside the GB single market and customs union and single market whilst nominally still part of the UK is probably the best way forward, with the caveate that this must gain and retain the consent of the people of Northern Ireland, regardless of DUP objections. Doing this against the explicit consent of the Northern Irish people WOULD be a violation of the good friday agreement.
Not really. As a summary with sources, here’s the relevant bit from Wikipedia:
“The Agreement also makes reference to the UK and Ireland as “partners in the European Union”, and it was argued in R (Miller) v Secretary of State for Exiting the European Union that the Agreement meant that the consent of Northern Ireland’s voters was required to leave the European Union (Brexit). The UK Supreme Court unanimously held that this was not the case,[30] but the Agreement has nevertheless strongly shaped the form of Brexit.
During the negotiations on Britain’s planned 2019 withdrawal from the European Union, the EU produced a position paper on its concerns regarding support of the Good Friday Agreement by the UK during Brexit. The position paper addresses topics including the avoidance of a hard border, the North-South cooperation between Ireland and Northern Ireland, the birthright of all of the people of Northern Ireland (as set out in the Agreement), and the Common Travel Area.[31][32] Anyone born in Northern Ireland, and thus entitled to an Irish passport by the Good Friday Agreement, will also be able to retain EU citizenship after Brexit.[33] Under the European Union negotiating directives for Brexit, the UK was asked to satisfy the other EU members that these topics had been addressed in order to progress to the second stage of Brexit negotiations. In order to protect North-South co-operation and avoid controls on the Irish border, the UK, led by Prime Minister Theresa May, agreed to protect the Agreement in all its parts and “in the absence of agreed solutions, the United Kingdom would maintain full alignment with those rules of the Internal Market and the Customs Union which, now or in the future, support North-South cooperation, the all-island economy and the protection of the 1998 Agreement”, with the acknowledgement that this is “[u]nder the caveat that nothing is agreed until everything is agreed”.[29][34][35][36] This provision formed part of a UK-EU deal which was rejected by the British parliament on three occasions.[37] It i”s now the position of May’s successor, Prime Minister Boris Johnson, that this so called “Irish backstop” must be removed from the proposed agreement.[38]”
https://en.wikipedia.org/wiki/Good_Friday_Agreement