I get the frustration, and I understand the official rules, but having flown both Air France and KLM codeshares with Delta, from the US to Europe, I would not be too alarmed.
From a pure logistics standpoint, when you check into your flight, it will be a Delta rep, the gate crew will be a Delta crew. Delta really does not have "scales" at the gates, and I do not even recall seeing those "sizing boxes" anymore. The point being, if it "looks" OK you will fly. They will not be measuring bags and tossing anything a cm out or a fraction of a kilo over. At 30 lbs though, I would encourage you to do a bit of triage and knock out 5 lbs or so, but for the sake of your back and ease of getting around Europe, rather than the airline.
Returning to the US though, whole different story if on a codeshare, then the Air France staff will apply Air France rules, so checking your bag on the way back might be a smart option.