We would take the approach that if we were the seller, we would ask the the buyer to open an eBay/PayPal dispute.
This way- the tracking and other information regarding the shipment would be in eBay/PayPal's hands.
You would be protecting yourself against any chargeback.
What good is sending another phone?
The physical time it would take for delivery would still be MORE days, till its arrival.
And, you have absolutely NO GUARANTEE whatsoever that the buyer will return the phone as promised.
This is a situation where you must take control.
You have sent the item. It is tracked, and insured.
You may not appreciate the buyer's inevitable NEGATIVE, or less than flattering feedback, but it appears you have done everything correctly.
Stand your ground. This is not something to be negotiating with the buyer- especially when they are asking that you send another IDENTICAL item, with the promise of returning the other.
ON BIG POINT- is it not VERY likely the ORIGINAL phone sent would arrive before ANY replacement?