[mothered]

I contacted CS again today for an update, and they said that the fulfilment centre has received the return, but they said "I returned a different unit of an item".

I don't understand their point, neither do they.

You've used the wrong Item received method, so obviously a different product has been returned.

According to the rep, i was supposed to send photos of the incorrect phone before i returned it, but i explained that the previous reps said just to return it (no photos). 

What's done Is done.

There's no point trying to explain It - It cannot be reversed.

Now they're launching a 3 day investigation, where i guess they're looking over all the details.

Should I try a different rep?

You can try, but they'll most likely refer to previous and current records.