I'm an artist:

You can add all your aliases here as separate artists and publish music on their behalf (as an independent artist without a label).

If you collaborate with other artists, you can specify them in your releases without adding the contributing artists to your Volumo account (provided that you have all legal rights to do this, of course, as you will be responsible for sharing your revenue with them).

I'm a label:

You don't need to verify any artists for music that you publish on behalf of your verified label.

Multiple artists & labels in one account?

Yes, but one account = one payment recipient (with separate sales reports, of course).