I think it would be. It wouldn't be an end all solution to hapiness, but it would remove a major source of conflict.
Yeah. As wild as the speculation is to make a conclusion here, I think everyone following the same cosmological ideology (or, as in this case, getting closer to doing so), that did not demand anything "unworldly" of them, would reduce unproductive conflict by a notable amount. There'd be fewer people following ethical systems derived from something entirely separate to humanity, and so they'd shift towards ethical systems based on more "worldly" things such as humanism or utilitarianism.
I'd think it would be agnostics who make up the majority of these atheists, as I think a lot people are strong atheists purely as a reactionary position to the prevalence of theism, and would be weak atheists, specifically agnostics, otherwise. That might just be wishful thinking on my part though.
Insisting there's nothing outside the visible universe is stupid though, as is saying we know a significant amount about what is in the visible universe. I'd hope it wasn't those types of atheists that dominated, since that type of rigid thinking we limit truth-finding somewhere down the line, just like claiming to know what's outside the visible universe does.