[mothered]

First and foremost, who's the company?

My question is can I fill the box with anything as long as it doesn't make sound and matches the weight?

Be sure It's a decent product of equal weight.

As such, If It's opened, the return can be justified.

And if they do open the box for some reason how can I return it? Am I stuck with it in that case?

It's not an Issue whatsoever.

It can be returned for a legit refund by saying you were In a rush, and mistakenly packed the Incorrect Item In the box. As to why It was resealed to perfection, you were concerned that an open/damaged box would void the refund, hence returned It In Its original state.

My final question is can it blow back on me in the future? E.g.
Someone buys it 6 months down the line and I'm getting rebilled and reported?

No.

Who's to say the next buyer Isn't a social engineer claiming he received an Incorrect product?

Do you have any experience with argos at all?

They check sealed box returns, namely In-store.