There is nothing in A50 that prevents any country leaving
Correct. Also correct to say that Article 50 does make reference to GFA as GFA is not written into EU Law. However, any EU member that has a border with a third country must apply EU border controls. Failure to do so could result in the offending member state being removed from the European single market.
This where ROI gets into a pickle. As an EU member it must respect EU law, but border control, as required by EU law, goes against GFA.
What type of border can be developed by ROI that satisfies both EU and GFA remains to be seen. However, it will need to be sufficiently robust to prevent cheap imports arriving in NI under WTO and finding their way into ROI undetected. Once in ROI such goods could end up almost anywhere in EU.
At same time border would have to sufficiently relaxed so the GFA requirements that ROI has committed to are maintained.
A hard one for ROI to solve. On the upside it may accelerate the reunification of the Island of Ireland. Might piss of the DUP, but according to some posters it is all their fault anyway?
Remaining in the CU and SM do that
That is same as remaining in the EU. 17.4 million voted leave, not remain.