I am not an expert, here is what I know or thought

firstly, best case is you will still get GC approval with no impact from the laidoff, according to some past cases I heard, that is not impossible

worst case is you can't find a new job, and INS ask you to interview or RFE, you can't provide employment letter, and they deny your GC, then you become illegal and have to leave..

There are many ways to avoid the worst case, until you are asked by INS for employment letter, you are safe to stay here, if you are asked, you can ask your current employer to provide that letter even if they laid you off, sometimes they will do that; or by that time, you might very possibly already found a new job.....since there are so many ways, I think the chance that worst case actually happen is not big

先说the worst case，之后不能找到新的相关的工作：
1．我是否在Layoff之后，还可以合法地呆在美国呢？
可以

在这种情况下，我想H-1B就会失效了，对吧？
对
2．如果可以呆，我将以何种身份合法居留呢？
AOS
可以呆多久呢？
unitl your 485 is denied
我可以自己Renew AP和EAD吗？
I think so
会影响绿卡的批准吗，或者说有什么Risk或Catch？
possible


再说一下the better case，找到新的相关的工作，但在Layoff的时间和新工作开始的时间之间有一个Gap：
1．中间的Gap可以有多大呢？在Gap期间我们必须离开美国吗？
hopefully nobody cares about this gap, or you can ask for a no-pay vacation to stay employed until you find a new job
2．我可以把H-1B转到新公司去吗？
可以, as long as new employer willing to sponsor h1b transfer
如果不行，用EAD可以到新公司工作吗？
可以
这样做有没有什么Risk或Catch？
if 485 denied, EAD will become invaild


3．AP和EAD可以继续Renew直到取得GC？
I think so

最后是the best case，在Layoff之前找到新的工作：
1．我必须把H-1B转到新公司吗，或者说我必须保留H-1B状态吗？或者用EAD工作？
no, you can use your EAD
2．继续用H-1B(transfer)和用EAD，对GC的申请和批准，有什么不同呢？推荐用哪一种呢？
H1B transfer is considered safer if you can get new employer to do that for you

again, I might not be correct, just FYI