Re: "The FA said, "So, you don't agree to not moving his seat with your knees? Then I have to notify the purser." The purser came back and turned what had at least been a subdued conversation, into a spectacle. She asked, "Are you going to stop moving his seat? As an Exec Plat you know he has the right."
The first conversation was subdued and you were asked to not move the person 's seat. If the tone was raised after your refusal to obey a subdued tone, then it sounds like it was justified.
There is no rule against reclining a seat. After all, that is what it is there for.
I suspect you problem was your inability to use your computer, rather than the story about your knees having nowhere else to go. You don't get to choose when someone reclines. And, just because he reclined early does not mean he was going to be reclined for all of the flight.
I can't believe you have never experienced people reclining before.
Do you never recline?