Can any Confluence user edit an InVision screen without having an InVision account?

If your screen would be shared using the Members and Guests link and you are logged with an InVision user member of the team is possible to edit the screen 

Can I embed screens with a free account?

Yes, users with an InVision account can embed screens in all available modes.

Can I embed InVision screens without an InVision account?

Yes, users with no InVision account can also embed InVision screens as long as they have the links to the screens.

The command /InVision is triggering a different InVision app.

It can happen that different apps are called typing the /invision command if they are installed at the same time in Confluence. Make sure to select InVision embed for Confluence from the dropdown menu. Alternatively, you can look for the app using the Macro menu.