If you call AO and the defender then grounds the ball, the question is "How did the ball get into the in-goal?". The answer is "From an attacking knock-on", therefore the outcome is 5m defending scrum.The 'mullered in the scrum' post brings to mind a recent thread about teams asking to play, or not play, advantage.
If a captain came up to you during the game and said "ref, we're getting mullered in the scrums. If they infringe, can you call advantage over if we've got any possession so we don't have to scrummage?" would you still go for the 5m scrum in this case, or call AO as soon as the defending player in in goal collected the ball?
And if you wouldn't call AO in this case, would you also no call it if the same thing happened on half way (i.e. the non-infringing team took possession 10m back from the infringement)?
This cannot arise in midfield because grounding the ball is pointless.