I am attaching a bathtub spout to a copper stub which is connected to pvc inputs. How do I get the spout to be tight against the wall when the plumbing has so much in and out flex?

Your pipes in the wall were either installed wrong or the backing has broken loose. There should be a short horizontal stud between the verticle wall stubs for the pipes to attach to.

Bob nailed it! Insert a short piece of wood to give the spout something to pull against, Good luck, Tom