Mitchell would almost certainly have replaced Duke if Charlie had been definitely out of action. If Edgar Mitchell had then somehow also been disqualified, then several other scenarios could have been in play:The mission would simply be delayed until Duke or Mitchell could recover - putting Apollo 16 much later into the year than April 1972. Perhaps a late summer or even early Autumn launch.
The other issue at work obviously would be choosing landing sites with the limited number of mission slots. Apollo 13 was to land at Fra Mauro and 14 at Littrow Crater. After 13 wasn't able to land, they decided to send 14 to Fra Mauro because Littrow Crater wasn't considered as interesting of a site. In addition, the Descartes region was considered a potential landing site but Lunar Orbiter photos weren't high resolution enough to determine if it was safe to land on (too bad they didn't use those leftover Corona cameras in LO like originally planned). They had to thus photograph it from the Apollo CSM and this wouldn't have been possible by landing at Littrow Crater. The Apollo 14 crew meanwhile photographed Taurus-Littrow, which was eventually where 17 landed.Apollo 15 was originally supposed to be an H mission and land at Hadley-Rille. After 15 was turned into a J mission, the landing site was moved to Hadley-Appenine. Choosing a site for Apollo 17 was more problematic. They narrowed it down to either Taurus-Littrow, the Marius Hills (suspected to contain volcanic cones), or a large crater, most likely Copernicus. Tycho was the scientists' first pick but considered much too dangerous to land near.The scientists also wanted the Schroeter's Valley/Cobra Head site, but that was not a viable idea since its location would make it impossible to photograph during preceding Apollo missions. The area was also quite rugged and the Cobra's Head itself inaccessible (the chasm is about 4200 feet deep).Keep in mind that scientists aren't flight engineers and wouldn't necessarily be aware of the risks involved in landing in certain areas of the Moon. They could merely offer a list of landing sites and the engineers/NASA administrators had the final say in where to land. For example, Harrison Schmidt had a number of unrealistic ideas about landing near Tycho or even on the far side of the Moon, a stunt that would require launching a separate comsat to maintain communication between the landing site and Earth. NASA didn't have the money for that post-budget cuts and besides, launching a comsat around the Moon would add more complexity and failure points--if the launch failed or the satellite malfunctioned, the mission would be ruined.