You'll end up with "professional 50s" when you introduce 15 and 25 m penalties. The time it takes for the umpire to move the player 15 or 25 down the ground gives the team more time to flood back with only 15 m lost. Just leave it and perhaps the players should just be more disciplined?
This is why it went from 15m to 50m. Players would scrag the bloke on the mark, get the 15 and then scrag him again to waste time without giving away too much territory.
As far as players saying "What!" or the like - players need to just keep it zipped. Since they brought in the practice of giving away 50s for 'demonstrative behaviour' there has been much less of it. (The Fletcher one isn't a good example because we didn't see what he did - however, you can clearly see that if a player talks reasonably with an umpire he is allowed to do so.) And there shouldn't be a difference between swearing at the ump or otherwise. If you draw that distinction, the players will just have a crack at the ump without swearing, which shouldn't be the case, even if the ump gets it wrong.
Players diving is a different issue and the finger of blame should be pointed at them, not the umps. Perhaps simulation should be 'discouraged' through suspension, frees against or players taking it upon themselves to not do it - another topic.
Two of the examples you used of players not giving the ball back - in both of them the player was having a discussion with the umpire without even attempting to give the ball back. In the second one the swans player wanted to jog back to the mark before giving it back - you don't get that option. Oh, and they seemed to be in the same match - you think the players would have learned by then.
Handing the ball back to the right player. This has to be paid a 50 each time, regardless of whether there is another opposition player saying it is his. Why? Because the players will try and milk it for all it's worth if they are given just a little leeway. "Oh sorry ump, I thought it was his free."
Agreed, the 50m penalty does need to be used as a last resort, but players need to ensure that they understand the rules themselves.