Now that PD is close to be current, and I am thinking of going back to US this July to stay and wait for green card. Can I go by using the B1/B2 visa or do I have to apply for H-4 visa?
-Not need to get the H4 visa, and just use the B1/B2 visa to enter U.S. Then, you can file I-485 if the PD is current. If the PD is not current, you can change your status from B1/B2 to H4 within U.S. before the B1/B2 I-94 expires (normally your B1/B2 I-94 should be 6 months).
what is the consequence if I overstay my B visa?
I used AP before and did not apply for AP before coming back last July.
-Because you used the B2 visa to enter U.S. last year , not used AP or H1/H4 visa after you filed I-485, your I-485 is considered invalid, so you need to file a new I-485 again when the PD is current.