If the sprocket retainer has let the sprocket slide along the driven shaft to me it would suggest that either,
A. You put the retainer on in a position where the the splines were aligned when you bolted it up.
You did rotate the retainer when you put it on,,,,,right?
Did you try rotating the retainer 180° to see if the sprocket still moves?
B. Wrong or stuffed retainer.
If it is the wrong retainer I would check to see if its the correct sprocket (in regards to offset) as well.
Your chain is probably OK as your soft swing arm has taken most of the abuse.
Its probably got alloy particles on the joints/seals so clean it, inspect it for wear on the pins where its been grinding the swing arm.
If rotating the retainer does not fix it I would replace the lot, chain, both sprockets and the retainer.
This is a known problem with the retaining plate design, as someone said above its not possible to fit the plate in the wrong orientation, unfortunately although correctly fitted this doesn’t mean the plate won’t fail!, the plate sits in a groove in the output shaft splines, it is rotated when the plate securing bolts are fitted to a position where the projections on the plate line up with the splines on the shaft, thus the sprocket is secured axially on the shaft, but you’ll note that the the sprocket isn’t locked tight on the shaft, it’s not supposed to be, it can wobble slightly, this small amount of movement allows the ends of the splines on the shaft to rub against the projections on the plate, this gradually wears them until they wear away completely, at this point the axial retention is lost and the sprocket is free to slide up and down the shaft, this allows the chain to contact the swing arm, hence the damage, so the fix is to buy a new retaining plate, paint over the damage on the swing arm to prevent corrosion, it’s extremely unlikely that the damage to the swing arm is bad enough to cause structural weakness but get your Ducati dealer or a qualified engineer to confirm that , the design is at fault here, it’s a cheap and stupid design, I’ve seen them wear through at 3k miles though most failures I’ve seen are in excess of 8k miles, when you fit the new plate apply copper grease, this will reduce but not stop the wear, anyone owning a Ducati that has this system would be well advised to check the plate for wear periodically, every 2.5k miles, if the plate is significantly worn fit a new one, they are approx £8 from Ducati.