It's the well known clipping thing. There are a few different causes. You're in Parallel Projection--switch to Perspective view, hit Zoom Extents and then go back to Parallel Projection if you wish. You're model is a long distance from the origin or there is some geometry at a long distance from the origin. Keep you model close to the origin. Hit Zoom Extents. Does your model get smaller with a bunch of empty space? That empty space contains some stray geometry that needs to be deleted.

First, post the model or the part of it. Second have you tried the Tesselation suggestions on Keyshot Forum? Do you understand what they are trying to say? Without any more informations we are not able to help in the right direction.

Beginners often miss the basics of the program. To move the endpoint of a line you must use the move tool not the select tool. With nothing else selected click on the endpoint of the line with the move tool and you can move it anywhere. You can type a distance, or a set of co-ordinates or simple drag it to where you want. The trick is not to preselect or you will be trying to move the whole line.