2. If there was nothing wrong with the laptop, then that should be their reason for rejecting the RMA. Not "you installed an operating system".
I don't understand why there is such an obsession with their margins. How does that matter to me as a consumer in any way? I am purchasing a product. I expect to be able to return it if it is defective. Period. I do not care why they lie and refuse to accept a return on defective product. I do not care how much or how little money they make. I care that they are an honest business I can trust. If you value doing business with companies based on how little money they make, you are welcome to choose to do business with companies by that criteria. Those of us with different criteria do not become objectively incorrect for using those other criteria for making our decisions.