1) No way to argue about how you were paid in China. Their turf, their rules.
2) For how you were paid in US, you Could make a Federal case out of it. Form SS-8, Determination of Worker Status for Purposes of Federal Employment Taxes and Income Tax Wi... can be filed with the IRS. The form may be filed by either the business or the worker. The IRS will review the facts and circumstances and officially determine the worker’s status. Takes at least six months.
3) You can also attempt to recover FICA from 'employer' : "Workers who believe they have been improperly classified as independent contractors by an employer can use Form 8919, Uncollected Social Security and Medicare Tax on Wages to figure and report the employee’s share of uncollected Social Security and Medicare taxes due on their compensation."
Most practical approach: Pay all taxes as if you were a self-employed contractor, then take actions as above to (maybe) recover something from Former employer. Reference:
https://www.irs.gov/businesses/small-businesses-self-employed/independent-contractor-self-employed-o...
Ref:
