Heather,
First I want to thank you for being awesome. I can remember being the same age as your child when I first told my parents that I wasn't a girl. I did not have open minded parents, but it sounds like L won't have the same experience I did. For that, I'm very happy.
When my body started to change I was really confused. I threw tantrums, cried and ultimately rebelled in any way I could find.
For pronouns, ask L. Your child may take time to figure out how they want to be addressed and they may flip flop, but the most important thing is to be supportive, patient and caring.
I'm not sure where you live, but since puberty is starting is there a change you can speak with yours or L's doctor about hormone blockers? It can halt the changes and give you and L both peace of mind and time. For binding, have you tried sports shirts? I know I wore them for many years. Since L is young and their body is still developing, restrictive binding can affect bone growth and organ development, but a sports shirt or something similar can hold down breasts, won't look odd and won't cause harm. Periods are more difficult. I was 9 or 10 when mine started and I was adamant that it did not happen to me because I was a boy. It took my mother a lot of time to get me to think of my bodily functions as something outside of my control. I thought that, through sheer tyranny of will, I could just not bleed. L will probably have an easier time of it since you are open and you have had talks about it in the past.
The most important thing is to keep doing what you are doing. It sounds like L is aware of something but at that age it can be hard to fully explain how we feel. Let L take the lead and don't push one way or another to put them into a binary box.
If you have questions, please ask. We have a lot of people here who are happy to help. Don't be shy about asking any of us, the ladies are delightful, the guys are laid back and our non binary siblings are fabulous. We also have a spouse and parent board. The ages are all over the place so we do have members who are parents, young teens and adults. A lot of folks here remember being L's age, for sure.