its a bit like an iceberg - no one really knows what was going on behind the scenes, but nothing anyone says is likely to come anywhere near the truth.

That's not really a very accurate description of an iceberg...
..............
The most plausible constitutional solution for Ireland would be a version of the Chinese principle "one country, two systems". It isn't so much the existence of a separate Irish political entity ruled from Belfast that's the problem, but the involvement of a third party across the water in England. My understanding is that that notion is very much consistent with how Martin McGuinness eventually saw things.