This is covered by the following issues, right?
https://github.com/helix-toolkit/helix-toolkit/issues/102
https://github.com/helix-toolkit/helix-toolkit/issues/103

Please contribute to get this corrected :)

No, but it should be possible to do in a similar way as we do for SharpDX/WPF
We started a prototype:
https://github.com/helix-toolkit/helix-toolkit/tree/master/Source/HelixToolkit.Win8

try the ShowViewCube property

This is issue #17. See the three old discussions. I am not sure what is the best way to solve this. Help wanted! :-)

If you want a partial torus you should base it on the MeshElement3D and override the Tesselate method.
To implement a complete torus the easiest way is to override ParametricSurface3D and just override the evaluation method.
Some math required for both options, but should not be very difficult.

Added #74

this is creative use of the HelixVisual3D :-)
Sorry I don't see what is wrong, debugging may be required.
Should the end caps have different colours? If not, a better implementation would be to create a single mesh including everything.

yes, you could create simple (flat) grass by adding a texture to a rectangle. For the chalk lines you could try adding rectangles/or a mesh with chalk texture slightly hovering above the grass rectangle.

You could also combine the grass, chalk lines, text and other symbols on the same texture (make it large) and simply add one rectangle. This is a more static solution...

Yes, I think the PipeVisual3D is best choice here! And yes, you must calculate the arc!

This is cool! You should write a blog about this. Are you going to publish it as open source? It could be included as an example in this project!