Working on deals without clarifying the issues in the WA would result in a ridiculous number of if.....then.....else options
Article 50 describes the procedure of leaving the EU, not details of what the WA, PD and future relationships should be as every case could be different depending on which member leaves. Article 50, as passed in EU law and signed by all members, has to operated in accordance with the signed version in 2009 as opposed to briefing notes prepared by the EU many years later.
Unilateral changes to the agreed procedures after the referendum took place is bad faith by EU.
There is a huge difference between the conditions of the Vienna convention and a party to an international peace treaty deliberately breaking it
GFA does not prevent either Ireland or the UK from leaving the EU. A no deal is not illegal under EU law and GFA does not form part of EU law.
GFA lapses instantly under no deal as not possible for NI to comply EU policies that have lapsed due to no deal. Nobody has to follow the rules of the club they have left.
Even if GFA did not lapse immediately, that NI can no longer follow rules that don’t apply anymore is sufficient under the Vienna convention to make the GFA void