Oh no, what a mama nightmare! You're not the worst mother in the world;  not even close! It sounds to me like you're a good mother, worrying about your little baby.
I don't have experience with this in quite the manner I think you're asking, but I did want to drop in and say two things.
First: the scar may indeed be invisible. I speak from experience. I lost a chunk of chin in 2005, and was glued instead of stitched. (I was already a frequent flyer in the ER, due to being unlucky and accident prone.) I have to trace the outline of the scar with my finger for anyone to notice, and even then, the have to be about 2 inches from my face. Your baby may heal in such a way that even you can't see where she was stitched. The human body is AMAZING in it's capacity to recover. Your baby is very young, her body is still growing and certainly ready to heal. I would bet that when she's my age, even she can't find that mark. Have hope!
Secondly: as a heavily scarred person, please, please, please don't lament in front of or with your little girl as she grows. The most damaging comments I've received on my visible scarring is "What a shame.', as if I'm somehow ruined, or sullied. There was an accident. I was very badly hurt. I lived through it. Now I win every scar contest I get into. It's still hard for me to think that way, because of the way society looks at me. Seven stitches on an eyebrow won't be nearly as extensive as what I got myself into, but if the mark shows, give your baby the gift of a little swagger - now, when I'm rocking with my scars out, I'm just thinking to myself that it's no wonder I've never been mugged because I am clearly a bad-ass. If she has a mark, and it's a big if, teach her to accept it for what it is - proof she's tough, can heal, and nothing else.
I'm so sorry you went through this! I'm sorry poor husband broke his toe, and I'm sorry you're feeling so bad about what happened. Be encouraged. :-)