Very interesting, thank you for sharing.
I think the major reason for the difference in rating is caused because playing online vs. over the board is very different. And players that are used to play online might be not as strong over the board and vice versa. But this is just my assumption without any data behind :)
I totally agree with you. If someone resigns, the game is lost for sure. But if someone decides to continue playing, even with lost position, there are many chances to draw or even win! So never resign!
Usuallly I am pretty lazy with Puzzles, but I tried all three Platforms. I think I also liked chesstempo the most. However the Puzzle Games on Lichess and Chess.com are also fun