There are no
endsong.json files present in the files I received
Spotify has unfortunately sent you the wrong files. Spotify has two types of file packages: a package with just some basic info which only covers one year (this package arrives around 3 days after requesting), and a package that includes over 60 files with detailed info and which has data from the minute you've signed up for Spotify (this package arrives about 2-3 weeks after requesting).
In this case, you got the first package, which doesn't include the data needed. The Spotify employee who helped you probably doesn't even know of the existence of the large package. Sending the same email again may help, since another employee will respond to your request. Sorry for the inconvience!