The Immigration and Nationality act as amended by Congress in 1996 law requires that only those who are authorized to work in the United States be hired. Employers are required by law to verify work authorization before employees can begin to work. There are clear regulations and documentation requirements and a form I-9 is required to be filed for every employee who gets hired.
Employment authorizations come in many forms. Everyone including US citizens have to prove work authorization to their employers before they can begin to work. For US citizens a declaration of citizenship, a social security card (unrestricted) and a driver's license, a US birth certificate/passport are some documents establishes their right to work. For permanent resident aliens their A number, SS card /DL combination or simply their permanent resident card does the job of establishing right to work. For other aliens the following are acceptable documents:
1) Original H1 approval notice,
2) Original EAD,
3) Refugee Travel document,
4) Asylum approval notice,
5) Other L or O status approval notice.
After AC21 was passed it became possible to change form one H1 status job to another without waiting for item 1) to be available. USCIS released regulations where the receipt notice would establish right to work for upto 240 days.
I am not sure why your lawyer and/or HR person are either in denial of the law or ignorant of the law.