"And kindly explain and justify beyond doubt your assertion that 'Del-boy's constant use of "plonker" for his brother firmly established the word as non-sexual'. Why/how did it?"

By constant and repeated usage in a firmly non-sexual context over many years in a pre-watershed programme watched by millions. The self-appointed arbiters of language have no control over that. When I were a little lad me mum and dad frequently put the only lp they had on the gramophone, a collection of songs by Peter Dawson, the first of which was A Bachelor Gay. The song laments the fellow's lifelong lack of success with the ladies, until:

"When he fancies he is past love
It is then he meets his last love
And he loves her as he's never loved before"

One can envisage many of a younger age puzzling how the chap can be both gay and living in lifelong yearning for a lady-love. That's language for you, evolving fast in response to popular usage in spite of the begrudgers, and long may it do so (hopefully -heheh!). How did "gay" change? By constant and determined usage in a new context, that's how. Just like Del Boy did with "plonker." In my view, it's a valuable addition to the mildly-amusing lexicon of gentle pejoratives.