NI is not a sovereign state. Call the territory by whatever euphemism you prefer, is a political division of the UK. That train left the station in '22.

Amended UK-EU withdrawal agreement? No.
24 Jan 2019: Barnier says time-limited Irish backstop not possible

Amended Uk-IE GFA after 29 March? Possible, low-probability. Besides this would require a referendum in both countries, no?
Amended UK-IE GFA on or before 29 March? No.

Why? The answer to all these questions is, no one trusts Tory gov or HOC, not even the dread Arlene the Paisley.

Diversity is the key to economic and political evolution.