Now I admit I am not 100% sure about this and I'm only putting my interpretation over here BUT!

Surely if an attacking player is deemed to be over the line but the ball isn't grounded which results in a Held up call "Over the Line" with a 10m restart then the same rule applies to the defending team?

i.e Body over the line but ball not grounded equals held up behind the line and drop out? Right call from Silverwood?

As posted on other thread I thought Silverwood had a decent game, at least he didn't turn it into a Penaltyathon and only the Forward pass at the end I would have any real complaints about.

It shouldn't have been a drop out, as if that was a try scoring oppurtuinity, it would never have been give as a try, thus should never have been a drop out. We see many times Full Backs diving into the field of play to avoid a drop out, will only their Head, Shoulders & The ball over the line, and this never gets blown for a Drop Out. It was a wrong decision in my view, and was cancelled out with the forward pass decision.

You are indeed correct, however, for the ref to be able to call held up over the line the ball must be over the line, the same applies for a drop out and awarding a try.

if Wellens had been the attacking player in that situation, it wouldn't have been given as held up - it would have been adjudged as a tackle in the field of play.

The ball wasn't grounded at the time the call was made, Wellens had the ball on his chest. If the ref calls held at that moment, then the ball is over the line and a drop out is the correct call. It was only after the tackle was called that he moved the ball onto the ground, thus making it look like the ball was in the field of play.