(Law 22 would have done, given the renumbering.)
[LAWS]18.26
[FONT=fs_blakeregular]The lineout commences once the ball leaves the hands of the thrower.[/FONT][/LAWS]
[LAWS]18.37
- The lineout ends when:
- The ball or a player in possession of the ball:
- leaves the lineout; or
- enters the area between the touchline and the five-metre line; or
- goes beyond the 15-metre line.
[/LAWS]
If the area includes the space above the ground, this is saying the lineout is over as soon as the ball leaves the thrower's hand!
If it doesn't include that, then the lineout is not over when the ball is caught inside the 5m area (but that is an infringement, or course).
The reason the lineout starts as defined must be that this releases the players for the long thrown-in, otherwise the lineout could start when the ball reached the 5m line. To make sense of the way the law is phrased, I think we must assume the ball has to be played into the area from the lineout. That means the lineout is over, and it is hard to see any case against playing advantage, if appropriate.