After H-1B transfer is approved with the new employer, how long do I have to decide if I can to join the new employer? Can I join after 90 days of H-1B transfer approval? Is there a deadline?

My employer ended my employment due to COVID reasons on May 31, 2020 but I never received an official termination letter. Calculating the grace period depending on the assumed termination date mentioned above, I have till July 30 to find a new job. Should my new employer give me an offer or start the H1B transfer application before July 30? What happens if they fail to do so in time? Can I extend the grace period on the basis of having an offer in hand or maybe because of the pandemic? My final interviews with potential employers are being withdrawn because of the time restriction.

I am having H4-EAD,Can I start any business?

I am a Registered nurse present in the US with F-2 visa stamp on my passport. My husband, being in grad school, has 2 more years of valid I-20. An employer sponsored me the green card, and recently I received my EAD based on I-485 application. But both my I-140 and I-485 are still pending. The employer wants me start working although my I-140 is pending.<br>
1. Do you recommend to work on EAD while I-140 still being pending?
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2. If I use that EAD, and by any change if I-140 is denied later, should I leave the USA to avoid unlawful presence? How can I come back to F-2 status again?