As requested, I'm posting this here in case anyone else has had this issue. I was able to download the APK, but when I tried to install it, I got the message "App not installed," even though I had set "Allow installation of apps from unknown sources" to ON under the security settings. I've found various reasons listed online for what can cause this error, but so far none of the troubleshooting suggestions I've found have solved the problem.
OS: Android Marshmallow Phone: Kyocera DuraForce Pro
If anyone has any ideas for why it isn't working, I'd love to hear them!
I dunno either *shrugs*. I tried restarting my phone and updating all my apps to the latest versions, but that didn't do anything. The only thing I changed before this time was I gave Play Services messages permissions to fix an unrelated issue.
I do know why it happened this time - the apk went from a signed apk to an unsigned one (there was a login issue with the signed one, and I can handle that issue later), and so it needed to be removed first.