IPhone – Can Find My iPhone password be different than the Apple ID

You can do it from Find My iPhone on another iPhone, you just need to sign in as the person whose phone you're trying to find [or get them to do it].

Alternatively, you can find all members' phones if you use Family Sharing.

I doubt there's any way to do it from an Android. That really wouldn't be considered safe.

Yes, you can have multiple devices linked to a single Apple ID each with Find My iPhone turned on, and they will be tracked individually. You can switch between showing different devices (or all devices) using the link at the top of Find My iPhone.