Regarding charging my phone, since I play music almost all day, do you see a problem with me leaving it plugged in then too, and not just at night?
If you leave your iPhone plugged in all day, there shouldn't be any reason to plug it in at night as it should already be fully charged.
This Batteries+ screenshot of my iPhone should give you an idea of what kind of battery life to expect playing Music from various sources. I don't see the need to leave your iPhone plugged in just to listen to music. In addition, I have seen MacBook batteries swell and damage the system when left continuously plugged in, and while I have not personally seen that in iPhones, it is the same battery technology, so why take a chance?
