And I can’t believe a company with as much experience as Douglas had with jets in the late 60s, would mess up a design that badly.

Douglas philosphy was that aircraft design should be evolutionary rather than revolutionary. The DC-10 uses a scaled up DC-9 designs for the leading edge slats and the trailing edge flaps. The fuselage structural design is similar to the DC-9 but scaled up to the DC-10 size. Many other systems and design features of the DC-10 follow this same pattern.

Did they mess up the design, they sold more than the “revolutionary” L-1011? Was the design safe, there are still many DC-10 flying?

I like to compare the DC-10 to a car. Say you were Volkswagen and you wanted to make a car as big as a Cadillac. You surly would not want to start with the “Beetle” and just make it bigger. Will that is basicly what Douglas did with the DC-10.