Hi, thank you.
1/ You can create basic roundabout from several T intersections. The nightmare roundabout is only possible if you have a custom Static Mesh for it. Some parametric objects may be added in the future.
2/ Right now it is only possible with custom geometry. Will work on it after I finish current update.
3/ OSM support is not planned for now.
4/ It is possible to replace road section with a custom mesh.
Best regards,
GlimpseOfDawn