I've assumed that the Vulcans get their mental abilities from the discipline of logic. The Rommies may have these abilities but can't focus them as effectively.
Sybok could mindmeld yet he chose not to control his emotions at all. We don't really know enough about the Romulans to say either way. Maybe JJ will answer the question for us.
Yes, but Sybok was also raised on Vulcan. He at least had the experience of their training before he chose to abandon it. It all seemed to come rather easy to him. And though it is just speculation, Sybok may possibly have been a standout even by Vulcan standards, pointing again to a combination of both mental training and even the most minor of evolutionary branching. We just don't know enough about him.
Some would argue that the mere millennia the two species have been separated might not be enough for such drastic evolutionary divergence, but they did split after achieving some not-so modest advances in a number of sciences, almost certainly including genetics. That manipulation coupled with their very distinct environments could have propelled the Vulcan/Romulan split a little faster than the standard Darwinian crawl. However, I suspect that the foundations for the mindmeld are in fact in both species. I personally believe even a human could learn the neck pinch if he knew enough about the victim's anatomy. (Vulcan telepathic abilities might just give them a narrow advantage now that I think about it. Seeing as they might be able to better "sense," even sub-consciously, where to apply the perfect pressure for the end result.)