This is happening in my realm as well. I have suffocated in walls in the nether and had to search for my items so I don't lose them. Or I end up 6000 plus blocks away from where I am supposed to be. This happens both going into and coming out of the nether.
I tried it as you suggested and put the sticky piston on the bottom and the regular piston on top. I still did not get the achievement.
No I did not stack them, I had them sitting on the ground with a sticky piston pushing and pulling the regular piston back and forth.
Here is a video clip of the minecarts stopping while being on a power rail.
Plus other redstone builds that do not work.
This was fixed with the new 1.2.5.15 Beta update.
I was able to go back to version 1.2.3 for Xbox One. When I tried to go into the game it spawned me in the Nether froze and closed.
It appears that the nether portals are bring you out at the exact coordinates that the portal you are trying to use is located. For instance if your nether portal in the over world is located at -6300, 57, 458 you will come out at that address in the nether and vice versa.