Under Body Air Deflector Skid Plate Shield important?

The positive battery terminal on my 2013 Explorer Sport (only 22K miles)
developed a lot of corrosion and ate through the ring terminal that attaches
to the post.  Dealer said it could not be repaired and the entire harness
($250+), plus almost four hundred in labor and a new battery for a total
>$820.  
My question is, "Could this have been repaired simply by replacing the ring
that connects to the battery, rather than the entire cable?"
Also, does anyone know whether the replacement of this harness would
involve removing the Under Body Air Deflector Skid Plate Shield, which
turned up missing after I got the vehicle home.  I had asked them to do an oil
change as long as the vehicle was there.  They removed my old oil change
sticker, but said they did not get around to the oil change.  I know this skid
plate must be removed to do an oil change, but they disavow having started
the oil change procedure.