[mothered]

I am completely prepared but also a little nervous. Last night I contacted support twice with different information just incase and I was told

"Once the product is opened. We are unable to accept the product as return. The reason is that we cannot resell them as new to other customers."

^ They're referring to open box returns, such as (for example) opening the box and returning It for a change of mind because you didn't like the colour.

Sorry I'm a confused.

If they do not allow opened box returns

If they open my box for some reason and find the cheaper phone won't they say I'm not allowed to return the real phone regardless of it was an accident saying that I opened it to put the phone in?

I've just quickly researched their terms, and they do accept open box returns:

If you've changed your mind and need to return an item, you have 30 days from the date of collection or from the date of delivery to return your item(s).

Your item(s) needs to be:

Unused and with all original components
In its original packaging (with the tags if applicable)
In a resaleable condition
With its proof of purchase
With any free items that came with the product

Source.

Other sites state the following:

Can You Return An Opened Item To Argos?
The answer to this question is yes – you can return an opened item to Argos. However, you’ll need to have the original packaging, and any tags will need to be attached. You’ll also need proof of purchase in the form of a receipt or email confirmation of your order.

Source.

Can I return an opened product to Argos?
Yes, you can return an opened product to Argos.

Source.