Is there a cable with which I can use my laptop as a monitor from my PC?
No, it does not exist
You could use an HDMI cable and try to split the screen
Use teamviewer. There's a special USB cable with which you can establish a computer-to-computer connection, the input and output are interchanged.
The laptop is wired INTERNALLY, its graphics controlled directly. So you would have to find a program that converts any signal input into the graphics output of the laptop.
I don't know if there's.
No, because the graphics connections on the laptop are usually just outputs. For example, you would need a capture card
