Can I pay cash for company social security?

No, social insurance is a law, just like taxation. If you don't pay, the company must pay and won't give you cash.

1, no need to return it.

If you are employed in any unit, any unit must pay social insurance for you. As I said, it's the same as tax. If the unit does not pay, you can report it.

Further reading: How to buy insurance, which is good, and teach you how to avoid these "pits" of insurance.