1. Which US national airline (not cargo or charter, not affiliated with a major) operates only DC and MD jets in it fleet?
Answer: Midwest Express. It operates DC-9-10, DC-9-30, MD-88 with services to BOS, LGA, DCA, ATL, DFW, LAX, etc. It also operates seasonal services to various Florida locations.
2. Which Regional US airline (same criteria as above) has the same distinction?
Answer: Hawaii Airlines. It operation DC-9-50 on inter-island services and DC-10 to the US west coast. Reno Air would be another regional airline until a few month ago. Now it's part of AA.