I'm not contesting checking a bag or bags...I do so all the time.

However, I'm a bit confused. Assuming you're flying in Y, your allowance is 2x50lbs for Europe. $285 sounds right for the third bag...it stinks and is expensive, but that's the way it goes. Could an agent waive it? Of course, but that would merely be a courtesy and it sounds like they're working within DL's baggage policy.

Past lost baggage does not have any bearing on paying bag fees for different trips.