Public buses seem to be aplenty. Only in Mahé and Praslin and not late at night.
From Mahé, the fast ferry to Praslin takes about an hour and costs around €100 return, or Mahé to La Digue (via Praslin) takes 90 minutes and costs around €125 return.
Motorcycles are not allowed for tourists.
It’s a good idea to make Mahe the last stop of your island hopping journey. After landing at the airport you should take advantage of the free shuttle bus to the port of Victoria. From there you can go by ferry (Cat Cocos) to Praslin and La Digue (traveltime: 60 minutes).