You don't pay CPF unless you have Permanent Residence/Citizenship so there isn't any mandatory saving for migrant workers (both low income unskilled and high income skilled labour) AFAIK?
Yep. As someone who worked on an EP, the difference was that I paid a low rate of tax that didn't contribute towards Singaporeans' retirement income, whereas a Singaporean living in Europe would pay a higher rate of tax that contributed towards Europeans' retirement income