Flying with 3 week old...help!

Okay, so my cousin was killed in a car accident on Friday night and we were very close. I need to make it to the funeral in Florida...but I have a 3 week old little girl. It’s over a fifteen hour drive so I was thinking of flying since I’m worried about having her in the car seat for so long and having to stop to feed and change her. The flight would be under 3 hours. My only anxiety is if it’s safe for her health to fly this early? I’m scared with the recirculated air that she could get sick. I also don’t have her birth certificate yet since it takes four weeks to arrive, so how do I get through security? Has anyone done this before? Advice? Opinions?