Arduino Uno R3: Directly supply regulated 5V to 5V pin?
Here's a less formal way to look at it: Providing power directly to the +5V pin is (almost) exactly what happens when the Uno is powered over USB. As USB power is perfectly fine, by design, then your setup should be OK too.

External regulated +5V can be supplied to the USBVCC net just as well, for example by using an USB-B cable.

As per the second question, you should not connect to V_in. NCP1117's output may then become an alternative, competing power-source and this is best to be avoided.