Hi,
1) Yes, I'm a native English speaker and often use "sprinkling" when referring to a light rain. That's the distinction. "Sprinkling" is only used when describing a light rain that would not soak your clothes. The type of rain where an umbrella may not be considered necessary.
2) Yes, it would be natural to say "It's raining outside" if you're indoors. For instance, you're at the office and hear the sound of rain against the office windows. You might say "I think its raining outside, you might want to check before leaving without your umbrella"