Players placing generator, hopper, chest and count goes up if they dont own the plot.

[primal hemlock]

i am not going to read all that. but you can simply check if the plot belongs to the player or not. if it does not belong to the player dont add the counter variable

[arctic sparrow]

You can use the can build condition to check if they can build at the block. Your current method doesnt work because other plugins can cancel the event after skript does its thing. When that happens skript thinks the plugin is not canceled

[versed cedar]

or you could just wait a couple ticks then check if the event is cancelled