First off, I am thrilled to see you doing so well!
Second, I am about 20 weeks and I still do not have flat or popped belly button. This is unsual in that this is my 9th pregnancy (3rd viable one). It does seem to have something to do with the amount of weight gain you have. As for the dark line, I have never gotten it. I am very light skinned and I do not tend to get the linea negra. However, this has no bearing on the health of your pregnancy, so don't worry.
as for the line... never get it... as for the button still innie, but when i laugh or something it will smooth out a bit.. i never get a completely smooth or outtie button, so i think that it depends on a buch of things??