5) The engineering departments of three airlines based in North America converted their DC-8 Super 60 series aircraft to the Super 70 series designation in house. Which three airlines did this?
Purely a guess, but on the assumption that it would take fairly well established and financed facilities to do this, United, Delta and Air Canada would be my first choices.
6) What airline operated the B737-200 into Lawton, Oklahoma?
Frontier comes to mind...