The issue with the border is really whether some other WTO nation complains that we're giving Eire/EU favourable treatment.
BBC article upthread suggested that exemptions are possible for instances of National Security - presumably intended to allow much harder borders than WTO normal - and that it could be argued that maintaining the GFA is a National Security issue so could defend a non WTO border between NI/RoI
Yep, there are various arguments that could be used to draw it out. No one knows whether they'd work but it would take plenty of time before a definitive judgement could be reached.
BBC article upthread suggested that exemptions are possible for instances of National Security - presumably intended to allow much harder borders than WTO normal - and that it could be argued that maintaining the GFA is a National Security issue so could defend a non WTO border between NI/RoI