I would completely ignore the dealer about the metal particles in the oil. If you look hard enough, you can find them in just about any oil change on any vehicle, especially with lower miles as there is more wear as the bearing get broken in.
Did the airbag light come on immediately after the airbag recall? If it did, the dealer should do the diagnosis free. AS for the repair, if the diagnosis relates to the repair, then that should be free. If it is totally unrelated to the repair done under recall, then they are entitled to charge for the repair.
If there was significant time between the recall repair and the airbag light, then they should charge for the diagnosis. If the diagnosis turns out to be related tot he recall, then they should do the repair free and refund the diagnosis, but go in expecting to pay for the diagnosis and repair.
Do go to an auto parts store like AutoZone but make sure to get the actual code. It will start with a letter followed by 4 numbers. It could also have a letter added at the end or in the place of the last number… The free code checks generally only include generic codes so if the CEL is due to a propriety code, then you have to pay a diagnostic fee at a dealer or an independent mechanic that has a paid subscription to the manufacturers codes.
BUT DO NOT let the dealer replace this engine.