What is my status right now?
-Your status is AOS.
Is it legal for me to stay in the US?
-Yes, of course.
Can I still apply for three year H-4 extension right now without any problems?
-You may need to go out of US to get an H4 visa, not enxtension within US anymore. But if you go out of US without using AP, your I-485 will be affected. Actually, you do not need to keep H4 status now, and your AOS is OK enough. If your I485 is denied, you can go out of US to get an H4 visa to come back as long as your spouse keeps his H1 status valid.