1- nXt doesn’t fully support the Rhino Rendered display mode at the moment but it will (hopefully soon). This is probably why you aren’t seeing an accurate preview for some materials in the viewport.
2- If the material you’re using has a texture map in it for the wood grain, you can change the texture mapping settings for the selected object in the Properties panel>Flamingo nXt section. The default mapping is a cube map but if you change it to by surface you can use the Rhino texture mapping controls also located in the Properties panel. If on the other hand you’re using a Flamingo procedural wood material, you’ll need to adjust the size of the grain in the properties for that material by double clicking it in the nXt control panel. There are usually a couple size values to adjust. Then to rotate the procedural grain, assign an nXt texture mapping method like planar or cube and adjust the rotation of the mapping widget. Click Edit Placement in the nXt mapping section to rotate it after it’s already assigned.
The next update to nXt should make all of this much easier as the goal has been to fully support the default Rhino 5 rendering tools. The current released version of nXt was designed for Rhino 4 which is why it’s confusing.