[mothered]

Is it a good idea when using the sealed box method, to replace the original item with a fake?

Yes.

In the event the box Is opened, there's a chance they'll pass It as the original Item.

What could be the possible consequences?

Nothing.

Simply say 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.

Where are good fakes offered?

AliExpress.

For example:
https://www.aliexpress.com/i/32568033437...4itemAdapt
https://www.aliexpress.com/item/32568043...8768%21sea
https://www.aliexpress.com/item/32568043...9491%21sea

Or this site for a wide range of phones.