Ok, I mulled this over my break and managed to get it working today.
It was indeed the permission on the folder, as execute permission is needed to traverse folders, read permission is not enough.
Edit: I confirmed this by editing /etc/passwd and then doing su logstash to test the permissions as the logstash user
I had previously excluded this possiblity since I tried a copy of that logfile with logstash given ownership but it seemed that the result of that test was probably confounded with other permission issues from the previous installation and therefore not sufficient.
(I feel like an error message for "permission denied" would be helpful)
Thank you again for your help!