How can I move two lower nodes of the rectangle to get x1 = x2?
One way to go: You could use
Path > Path Effects > Perspective/Envelope, check
Mirror Movements Vertical, hold
Control-Key while pulling the corner point with the
Edit Path by Nodes-Tool.
You can entry manually the coordinates of each node.
Select the first node:
Input a new X, e.g. adding 30 units to the existing value:
Then, select the other node and remove the same amount from the existing value of the X coordinate:
Now, you have a right trapeze: