TV REPAIROK, a happy end for a change
Flipped the melted diffuser upside down, refitted the LCD screen, fired it up...... did the trick, white dots are gone now
Fourth LED strip still working intermittently though.
Turned out to be the LED board to board connector at the power board (again).
Already had a failed contact on that connector for strip n°2, ran wires going through the chassis, soldered directly to the power board, to bypass the two faulty pins of the connector.
So I would have needed to remove the LCD again to run 2 more wires to bypass the pins for that second failed strip.... no way.
Friend sensed how pissed and mad I was, so he proposed to leave the LCD alone, and instead try to twist/bend the two faulty pins a bit. Seeing as the problem is the female part that must be loose, he twisted using tweezers, the spade / male part (on the power board side). Good idea, that fixed it, no intermittent contact anymore, backlight now 100% good !
Of course that can fail again later but that won't be my problem any more.
So we put it all back together 100%, one last check, twisted / tortured the TV a bit, still a solid backlight, good.
Friend loaded the TV in his car, going straight to his buddy who owns the TV, for a drink.
Friend gave me 50 Euros, so minus the 14 Euros I spent for the new LED strips, that's 36 Euros gain. A miracle, it's zero usually.
He said he will try to extract a bit more from the owner and if successful, will pass it on to me.
So, I call it a good day today. TV is fixed, got some money for my work, and TV is out of my bench and living room, less clutter... I can see the floor tiles again now and can go to the loo at night without banging my legs on that TV. That's good.
EDIT : just got a call from my friend, he arrived at the owner's place. TV still works, it survived the transport. Talked a bit with the owner, he said he would give me 50 more Euros. So 100 Euros total, minus 14 Euros for the new strips, that's 86 Euros gain for one repair. A history first, almost worth the trouble...