First, I fixed your model and the scene. I turned off the extension lines (at least temporarily) and updated the style. I also set the camera to Parallel Projection and made a top view of the model followed by updating the scene called “scene”. After saving the changes to the SKP file, I updated the reference in LO and reconnected the viewport to the correct scene. (How am I going to convince you to quit modifying viewports?)
Next, I rendered the viewport in hybrid so I have vector lines to look at. And I drew a temporary horizontal line in LO. I made it red to make it easier to see. That line is snapped onto the endpoint of the edge in the model we’re using for alignment. I also turned off grid snapping in the Arrange menu.
I moved the rotation point to the intersection of the red line and the end of the edge in the viewport, grabbed the Rotation handle and rotate the viewport. It’s much easier to rotate if you drag the handle away from the center of rotation.
If you want the extension lines to show in the style, turn them back on in the style in SU, update the style and save the changes. Then update the reference in LO. Display features like extensions just get in the way while modeling. Save them for the final display tweaks if you want to use them.