That first idea would be better imho if sabotage was only "declared" by a player and then "bought/discuss" in auction by everyone.
Here's how it would work :
1. some player go to the Land auction room (renamed "Land auction/sabotage" room).
2. he goes to a plot that don't belong to him. By pressing action button, he declares that one to sabotage and pays 150 $ (cost to be balanced). Only one sabotage by plot, only one sabotage by player. A plot to be sabotaged will be mark by a little bomb icon on the top right of the square.
3. at the beginning of production, a sabotage auction phase begins for each plot that have been targeted that way (screen returns to main map to show which is current plot before next auction begins).
During that auction, three players choose a price they're willing to pay to call for sabotage, and one player (the plot's owner) choose a price he's willing to pay to cancel that sabotage. The winning player of this auction is the only one to pay.
4. If the plot's owner won, no sabotage. If any other player won, sabotage occurs : MULE is destroyed. If there's nothing on that plot, that plot is lost.
EDIT : and if no one won the auction, or if someone managed to raise that price high above any budget, nothing happens, sabotage is canceled (the one that initiate sabotage still pays 150$ though).
Benefits of this new game option :
- it could reverse a crazy situation where only one player managed to screw all other three. So there would still be a lot of tension until the very last round.
- it could reverse it for many players at many rounds.
- it could release pressure on an overflooded market by instantly create shortage of some sort.
- you could use of diplomacy in that game !! Youhou

by suggesting others to fight that lead player, you can take over the game, and still never pay any sabotage yourself.
Big problem :
- MULE stops being a parallel competitive game and starts being an open war.
But I think it still make sense anyway