Solved on discord's server:
The issue with the path was related to scene tree order. The ViewportTexture tried to get Viewport
but the viewport wasn't added to the tree yet:

So put the viewport node above the node that uses a ViewportTexture pointing to it. Don't forget to mark those textures as local_to_scene
.
The problem with the labels were that OP was trying to modify original scene expecting to make individual instances be modified as unique things. Scenes are like the base structure for those instanced nodes, so modifying the source modified those instances too. The solution was to go to were instanced nodes were, right-click them and select editable children
. This way you can modify instances individually