It appears that much of confusion (grey area open to interpretation) comes from the definition of a ruck which is: "A ruck is a phase of play where one or more players from each team, who are on their feet, in physical contact, close around the ball on the ground." The phrase "close around the ball" is quite ambiguous. It suggests that you can form a ruck and then drive over the ball but you can't form a ruck too far in front of the ball to stop the opposition competing (the stonewall that Blue identified). But how far in front is no longer "close around the ball"? Example 5 by Dam0 seems to raise the problem squarely.
Depends on whether "close" is a verb or an adverb. I'd suggest it's a verb as there isn't another in that definition but it's definitely ambiguous.