Here are some answers

My~~! you only have 3大疑问盘旋脑海 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)根据老师讲课,使用EAD后,我的身份从H4变为AOS,若I-140被拒,我即失去身份,需重新转回H4。那么,

一则,我以什么名义申请H4——延期、转换身份还是…?
Use LG's H1B approval notice, go out of US and get a H4 stamp.

二则,我一定能留在美国申请还是有可能要离境?
travel outside.

三则,I-140被拒与我递交H4申请之间有无时限规定?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批准之后I-485Pending期间,H1失业,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卡工作有限制吗?

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.
变成AOS后又不工作了,会失去身份吗?还是通过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.