I have found the Tauon Music Box player, which seems to have the features close to what you ask for. Here's how it looks:
Below I will try to address each point in your question.
I would like to be able to play music from local files and stream from Spotify.
You can go to MENU → Settings → Accounts → Spotify and follow the setup instructions that allow for the importing, searching and playback control of Spotify music. It should be noted that this feature only works with Premium Spotify accounts (I don't have one so I haven't actually tested it).
Ideally, cross-fade between tracks is supported (as it is in the official Spotify app).
The player by default has cross-fade enabled. You can find the fade settings in MENU → Settings → Audio.
I would also prefer if songs are added to the queue by default. If the queue is cleared when someone clicks on a song, it is not ideal for a party setting where multiple people add songs to the list on the same computer.
The player doesn't work exactly like that, but it's close. You can add a song to the queue by using middle-click (alternatively, you can use the right-click menu or the Alt+Q shortcut).
If you have added some songs to the queue and you double-click a song in the library, the queue does not get cleared. Instead, the double-clicked song will be played right away and when the song ends, the songs in the queue will continue playing from where the queue was left.
Bonus features include: playing songs also from YouTube, maybe even with the video shown in a second window, so it can be displayed on a second screen.
Unfortunately this is not supported nor the player has a feature similar to that.
If you like the above and you think that the player fits your needs, you can install it from Flathub.
