I know this doesn’t help in your case but for others who came to this post whilst googling a way to open a newer version of the file and don’t have access to that version, there is a file conversion service: https://www.stokesdesignproject.com/down-save-indesign-file-service.php

I know it's a bit late but thought this might help people who stumble across this in the future.

If you don't have newer versions of InDesign there is a service which does this down saving CS5.5, CS5, CS4, CS3 files to which ever format you need for a small fee and can save you a lot of time.

https://www.stokesdesignproject…..ervice.php