Here are some answers

My~~! you only have 3大疑問盤旋腦海 after you read PJ's blog. It suggests either PJ's a good writer or you are very bright. And I assume both.
Before PJ returns to you, I'll get some answers for your puzzles.

(1)根據老師講課,使用EAD後,我的身份從H4變為AOS,若I-140被拒,我即失去身份,需重新轉回H4。那麽,

一則,我以什麽名義申請H4——延期、轉換身份還是…?
Use LG's H1B approval notice, go out of US and get a H4 stamp.

二則,我一定能留在美國申請還是有可能要離境?
travel outside.

三則,I-140被拒與我遞交H4申請之間有無時限規定?算不算身份上的Gap?
Once 485 is denied, you need to leave. A few days gap is neglected.

(2)若I-140批準之後I-485Pending期間,H1失業,我的AOS是否失去身份?
No, you will be still AOS, regardless of the loss of H4 accordingly.

(3)用EAD卡工作有限製嗎?

As a spouse of primary applicant, you can either stay at home, or use EAD to work. For the spouse, there's no job field limit.
變成AOS後又不工作了,會失去身份嗎?還是通過Renew EAD卡就可以保持身份?
No, EAD is not a status, you will be still AOS. Renewing EAD does not affect your AOS in your situation.