One piece world travel How does one travel across different blues in one piece?

Consider this for example:

A person from north blue wishes to go 

1: West blue- but that means he will have to cross calm belt and then grand line (new world) perpendicular to the current and then again calm belt. I understand that navy does it with its special seastone ships but how does normal people or traders do it?

2: east or south blue: if you travel through north blue, you will reach red line ( reverse mountain), which will launch you into grand line (Paradise). Again you are surrounded by calm belts, how do you get out?

Or

You travel the other side and reach Mary Geoise. But that point will again be surrounded by calm belt, being the passing point between 2 halves of grand line. So you cannot reach it, unless you are actually on the grand line(paradise side). Also, the grand line has a current to it, which means you cann not enter the paradise half ( east-south grand line) from.new world ( north-west grand line). 

Thanks, hope I am clear with the question. :)
If you read how the reverse mountain works, you will find out that the top of the mountain has 4 paths towards the surrounding seas and 1 path that gets you to the Grand Line.

I made a aproximate map of the OP world here OP MAP some time ago and I had studied quite alot about how the world is connected in One Piece.

Good question anyway :D