Sneaking into Delta One on relatively empty DL transatlantic flight
While there might theoretically be a crime - little to no chance that any prosecutor would mess with it - assuming this isn't a pattern and there is nothing more to the story.
Would the airline sue - also highly unlikely
If caught and appropriately sheepish that would likely be the end of it. Caught doing this again might prove different. The airline can cancel your FF account and delete your accrued miles. They can also decide that they would prefer that you take your business elsewhere and blacklist you.
You got away with it - once. Some events in life are best treasured privately.