As for the Android installation issue:
I just investigated the published APKs for this game and Fur Feed Battle and it’s because they both use the same App-ID and hence are considered to be the same application by Android package manager.
To fix this look for some option called “App-ID”, “application ID” or “package name” in Game Maker Studio options. It may by under Android-specific options or general options (since the same value will also be used for Linux XDG and macOS app packaging) and will currently be set to “com.company.game”.
Reasonable values would include “ziul-walls.matchfur.fatness-madness” or “io.itch.ziul-walls.matchfur.fatness-madness” (it’s conventional to start the app-ID/package-name with the reverse of a domain-name you control, although not strictly necessary).
Keystore most likely refers to the signing certificate – it’s completely OK to use the same certificate for all app, just keep it private to prevent others from publishing updates under your name. Build mode is generally just about whether the APK contents are obfuscated and optimized for size (release) or not (debug).
If you need any further tech help let know, I like your games after all. ☺
- Kuri