In the end, I don't see how their circumstances differed that much...and certainly not enough to explain the sizable gap in their career averages.
Your points are as ever well made.
But my point was simply that there were some very fine bowlers outside the West Indies that he had to face, and probably a higher standard of bowling overall than Hayden's had to face (at least in the productive part of his career ie in the last decade). I know you may disagree with me on this.
As for explaining the differences between their averages, I'd offer the following thoughts:
1. Hayden had the chance to bat against Bangladesh and Zimbabwe. Greenidge didn't; nor did he play against the minnows of his day, Sri Lanka.
2. The pitches in the 2000s have, as a general rule, been better than they were in the 70s and 80s. It's much less common now that an opening batsman will have to bat on a pitch with real seam movement or pace than it was in Greenidge's day.
3. Bats are significantly better now than before. I'm unable to explain how or why this improvement has happened, but it seems a pretty widely accepted fact.
4. Boundaries are shorter now than they used to be. Many grounds are now smaller than they were to allow for bigger stands. Moreover boundary ropes are placed some distance inside the playing area to allow fielders to slide around without hurting themselves on the pickets / hoardings / spectators.
Points 3 and 4 are particularly significant for players such as Greenidge and Hayden whose method was largely based on hitting the ball hard to the boundary.
For these reasons, as Mike Atherton put it in today's Times, "50 is the new 40".
Oh, and:
5. As a devout Christian Hayden had a hotline to God and therefore an unfair sporting advantage.
Who was the better opener? I wouldn't like to say. I went for Greenidge but largely on the basis of point 5.