Hey, I would suggest to make available the mockup editing on photoshop online. It would permit to edit it quickly while needed, especially if we don't have access to a supported device for the app.
I might have missed something, that's possible, but i did not find the feature. On the app we usually double click on the layer so it open a file we can edit to apply it on the mockup. Here, doesn't work. This would be good !