That's been in flux recently! Patch 6.81, which was active from April 2014 until a few weeks ago, devolved into a "deathball" meta which tended to end games early. (This was responsible for the rather anticlimactic conclusion of The International this past July.) Patch 6.82 introduced some new "comeback" mechanics, which generally enhance the ability of a team that's playing from behind to recover after taking a teamfight, or finding high-value pickoffs. The details are still being tweaked, but so far the result does seem to have been less one-sided games, both in amateur and pro games.

And yes, "team stealth" is referring to Smoke of Deceit. A really interesting item, available in limited quantities, which allows a group of players to become temporarily invisible and sneak around the map -- even under sentry wards, which usually reveal invisible heroes! -- until they close in on enemy heroes or towers.

maybe in the pro games, but even then that's a pretty iffy statement. a bad engage in a teamfight or surprise gank can swing things pretty hard if the teams know what they're doing. in anything below, say, high diamond there's just too many mistakes to make this claim.

For example, in all of the finals of the recent International, there was a leader at 10m who won the game. I'm not going to go through all the games and check, but I'd guess we'd find this to be true the vast majority of the time in professional DotA games, as it is in League games.

Not to make too sweeping of a generalization, but I expect it is inherent in games with an economy and relatively low random factors. On top of the normal expectation that who gains a lead is likely to be more skilled, economies that allow you to build additional strength via the dominance conferred by your current strength reinforce existing leads.