Ah, I had a feeling it did. This was the one information you didn't mention before that explains everything.
As you said, it "displays beautiful but turns horrible". One thing you should keep in mind about most train objects in BVEStation is that
they are not centered. They are built negatively for a reason related to BVE, and so that's why you see them turning the wrong way in openBVE. Objects must be centered for them to turn properly in openBVE.
Here's what I want you to do:
1) Keep that default, unedited extensions.cfg file. There is nothing wrong with it, and you do not need to change anything.
2) Go open both "R40Frontward" and "R40Backward" (I believe those are the .b3d/.csv files), go to the bottom of the page, and insert this same code I wanted you to insert on the R40_B:
TranslateAll 0,0,9.2375
3) Make sure the fifth line of the Car section in the train.dat is also 18.475.
Let me know if that fixed your problem with the turns.