There's another way in and out of the Grand Line that wasn't mention in the thread so far. Through Mariejois. Apparently Mariejois is somehow connected to the four Blues, likely in a similar fashion to Reverse Mountain. This information comes from Norland's flashback when we're told that the king had to ask Mariejois for permission to cross to the Grand Line. We're told the same information in Sabaody arc as well. Granted, this way isn't for pirates and the likes but for government affiliated ship and citizens.

It's also possible that the Reverse Mountain works the other way around, too. Who knows. Not a plot hole until Oda adressed the issue.

Another thing. Crossing the Calm Belt isn't a guaranteed death. Yeah, sure, it's infected with Sea Kings but it doesn't mean they will attack any ship crossing the sea. Not to mention, not all Sea Kings are of the same strength. Take Krieg's fleet for example. He had a fleet of 50 ships. That's a lot of fire power. I'm sure he could scare off one or two Sea Kings and be lucky enough to survive through the Calm Belt.