in regard to all of your hardware mechanical issues such as buttons and creaking, i think it is appropriate to expect such from a preowned phone.
When using the dark theme, cuteTube uses the same background image as the stock music/video applications. The image has a gradient. From your description, I would say that your screen suffers from colour banding, probably caused by something similar to the purple tint issue that affects my N8. FWIW, I don't think that any screen that exhibits such problems can be considered 'normal'. This is a premium priced phone we are talking about.