If I'm on a DONE4 and the flight I want is D0, any OW airline that would refuse to put me in an available economy seat has clearly lost the plot and has no clue what customer service is all about. They may well do it (and I'd be interested in anyone's specific examples where it has happened to them) but I would still complain loudly and vociferously if it ever happened to me.
No they haven't ;good customer service does not involve pandering to every whim of a customer regardless. If there is no availability, then I cannot see what there is to complain about when the airline provides that which is paid for
I do know of people of a DONE4 wanting to travel on a flight which was listed as D0L0 and not being able to book on it
Dave