It was already explained that when entering the NW, there can be different combinations of islands. The path to Raftel is based on the islands you visit, it's all a matter of finding the right combination. Then there is the Pose who can show which path is the safest, and because of that many will try to take the safest path,but not Luffy who takes the dangerous one, because it's more fun.
And in order to even make it past those islands you need to defeat the Yonko, so it's not like it's open to everyone to reach.
You seem to forget that Shanks was part of Roger's crew, as such he should have a good idea of how to reach it. However he didn't go after it because he most likely doesn't want to. He already made a bet on Luffy and the young generation of pirates.
That's really not how teleportation works, if he were to cheat like you said he could end up in the middle of the water, and since he's a user he would just drown...He needs to know the location beforehand.
Regarding Kuma... ending in the middle of the sea is likely, especially if he tries to teleport "10 km away, east". That's why I said he needs to to teleport with a small boat
But Kuma is really only a mere teaspoon of sea water in the ocean of the incredibly handy abilities oda created and will create, abilities that could be used by the craziest and most determined persons to reach a single regular island, even if it's surrounded by the harshest conditions ever.
Because they are pirates. Ok, better than real world pirates, but pirates nonethenless. People like Luffy, Rayleigh and Shanks are rare gems, the rest of them is the lovely scum of the seas we all love. And some of them are also powerful and sharp.
So, we have a single island, Über handy abilities like it's raining and realistic pirates, realistic pirates everywhere. And it's like 25 years that no one can reach it, and before that it's likely that for centuries no one did. And of course Yonkos were not always here to stop the rookies.
Oda is a great writer, so it's highly unlikely that Raftel is still a mistery because every single person that could have cheated has done otherwise because "honor", or because he was suffering of Uncanny Dumbness attacks.
If you ask me, even if I can not reach it I would try to mount on something like a blimp to see it from a distance and drawing it's shape and an approximative map of its location, thus making tons of money and gaining fame.
I do admit that Shanks may be able to reach Raftel but he's waiting for Luffy, he's one of the special cases, and I extend this status to any single Gold Roger crewmate still roaming the sea, with Buggy as the only exception.
Let's talk about the sea currents. From the four seas they go up the Reverse mountain into the Grand line, surrounded by the calm belts, where there are not sea currents. So, if all the water in the oceans go into the Grand line, from where no water can escape... How come that the four seas are still there? The water that keeps this pressure system on and working... where does it come from, if the end of the Grand Line is depicted as surrounded by the Calm Belt?
I repeat myself: To me Raftel exists, but it's simply the greatest mistery in One Piece World... Or else, it wouldn't have been a mistery for all this time. A mistery so great that if you go at the end of the Grand Line, you find no Raftel at all.
EDIT: Just read the bit about Mariejois National Treasure and One Piece: I agree with you on this, also for me they seems related.
