As for your other questions - your files will remain in tact, but accessing them is another story. If you have files stored on Creative Cloud, you will need to remove them - you will be downgraded to the free 2 GB size. Saving back to CS5 would work with Photoshop (maximize compatibility) and Illustrator (Save as to older version), but InDesign wont go back smoothly. You'd have to save back to CC2014 (which is no longer available) then convert back to IDML from there. CC 2015+ IDML wont open in CS6 and back. As for opening in other software: InDesign files are proprietary and there is no other alternative to open native INDD files than InDesign. Photoshop files can be read by some other freeware software including GIMP or Paintnet. You could also convert them to layered tiff files which can be a pain but will be available to more applications. Illustrator can open in Acrobat reader or GIMP again.
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