H1B petition approved-travelling on H4-Changing employer in US


I have an approved H1B petition from current employer, but due to lack of projects I am planning to go on H4 visa (husband is working on H1B).

I wanted to know:-

  1. If I decide to change the employer once in US, can my existing H1B petition be utilized for COS process (i.e not having to file a fresh H1B)?

  2. My current employer is not likely to give me I-797 as my travel will be on H4. Would this be required during COS? I have the I-129 receipt number.

  3. What timelines should I look at for COS process completion?

Please note that I will be travelling to US for the first time.

This is urgent. Please let me know.