Recovering data from corrupted SD card formatted as internal storage (Marshmallow)
1) will rooting delete the encryption key so that the card can't be
  accessed?


No, actually "rooting" will not delete anything. Rooting is just adding an application for superuser or root-level access. That being said, in order to root your device you must have an unlocked bootloader. Unlocking the bootloader will wipe the entire device, including the encryption key, and all other data on the device. On the Moto G 2015 there is no way around this. Oh, and removing the card and performing the operation will not work, once it is rooted, the system will be different and the encryption key will change. 


  2) is there a way to get fsck working?


Not without root access. To be honest, even if you did get it to run the chances it would correct this type of problem are very low.


  3) any other ideas for getting the media off this card?


Unfortunately no... once media has been adopted by the system, it is encrypted with a unique key to that system. If you can access the key, it is possible to access the data via Linux assuming the data is not corrupted. If the data is corrupted the chances of recovering encryped, corrupt data are extremely small. 

As much as I would like to be able to give you an answer to recover your precious photos and videos, I don't believe there is one. All I can say is that moving forward I suggest NOT using adopted storage if possible (if this card was simple external/portable storage, the chances of recovering media from it would raise exponentially) and use a cloud backup solution like Google Photo which has the ability to maintain a backup of your photos and video automatically.