HI D.,
As long as the swaddle is away from her face, I'd let her sleep on her belly. And I agree with the other posters... may be time to transition from the swaddle or try swaddling under her arms... With our first born we followed back to sleep and she was not a great sleeper. Could have been a coincidence. As soon as she was able she slept on her belly.
Our son was born early and slept on his belly in the NICU and on. He had some developmental delays and when I talked to the physical therapist about concerns I had about him sleeping on his stomach she told me that they actually see more kids with delays because they are put on their back too much. It was her opinion that the frequency on SIDS and the back to sleep movement was due to thick mattresses and blankets in beds with babies. After that I never worried about our son another day. He's perfect now and has been a fabulous sleeper.
Just a side note, and not one to concern you more... you said she's a great sleeper and sleeps for 12 hours straight. Be mindful about her always sleeping on the same side on her head. We hear a lot about babies who are on their back a lot developing a flat spot on the back of their heads, but babies who sleep on one side of their faces can develop something called tortacolis (spelling?). This can affect their necks and cause problems in development as well.
Good luck and joy your sweet little one!