Think of an Alexa-enabled WiiM device like an echo - an echo is tied to the account under which it’s registered, so a WiiM device is the same in practice ie it’s tied to the Alexa/amazon account you linked it to in the WiiM app.
For Spotify, that doesn’t really matter as it’s quite “promiscuous” and will cast to anything that supports Spotify Connect.
Amazon music doesn’t really deal well with multiple users accounts when it comes to Amazon echos, so it’s much the same with Wiim devices - you can only cast (via AlexaCast) to a device that’s on the same account as your music account. In fact, a WiiM device is probably weaker than an echo in that you can’t change profile as you might do with an echo in an Amazon household setup.
However, if your WiiM device supports Chromecast (which the WiiM amp does), you can use that to play from any Amazon account on your phone to the WiiM, albeit at best in CD quality.
Don’t know what you mean by priority- both the mini and WiiM device would need to be on the same amazon/alexa account for you to group them in the first instance.