company closes down

My current company will be bought soon by a company from Canada which has an office in the states. I filed my I-485 last July and my I-140 was approved early last year too.
1. Can I work for the new company (a foreign company) and still keep my GC application valid?
2. If yes for 1, do I need to file AC21 even my company wont revok my GC application?
3. I plan to renew my EAD card soon although it's never been used in the past year. Can I use it after I get the receipt or must I wait for it getting approved?

Thanks in advance.