As a former amateur goalkeeper myself, it boils my blood to see goals creep in at unmanned posts from corners. These are things you can control. So, control the controllables. Worrying about playing fellas onside is nonsense. It's a corner. They're all onside. A properly-marshalled defence can deal with second balls after any initial clearance. But the men stay on the post until that ball clears the penalty box. Being outnumbered is also a nonsense. No team is packing the box with almost every outfield player unless it's injury time. They always keep at least one back - and others will be outside the box. As mentioned, a well-marshalled defence has a chance to cope with that - but nobody can cope with unmanned posts. If the ball ends up there - as if often does from set plays - it's a goal. A preventable goal.
Any time I see a goal creep in at an unmanned post, I put the blame squarely on the goalkeeper. He is not dominating his area. I doubt managers - who know next to nothing about goalkeeping - are dictating this in most cases. Maybe people like Benitez - who think they know everything - are. But in most cases, I'm sure goalkeepers - and their coaches - are deciding what the course of action is in these situations. Same for free-kicks. If a wall does not do its job, it is almost always the fault of the goalkeeper. Yes, there are cases where an idiot ducks or pulls away from his team mates leaving a gap and selling his goalkeeper down the river, but the keeper must set it up correctly and choose the right position on the line to deal with the ensuing shot. Anybody ducking should be royally bollocked and be left in no doubt where the finger of blame is pointing. This is where the likes of David de Gea fall down. Too quiet.
Part of the art of goalkeeping is realising there are idiots on your team who cannot be relied on to stay on posts, stand on a ball having conceded a free, or stand up tall and stay tight in a wall. It is the goalkeeper who ensures these morons are not part of his first line of set-play defence on a post or in a wall.