I have to  go back and calculate the basis of the IRA because my client did his own taxes and has been including the conversion amounts as taxable distributions. His 8606 is incorrect.

1- Do I have to amend his prior returns and 8606 to get the correct basis?

2- In order to calculate the basis each year, what documentation should I request? to determine

---- IRA Contributions

---- IRA earnings

3- The IRA Contributions that were never taken as tax deductions and converted to Roth are not taxable?

4- Are the IRA earnings that were converted to Roth taxable?

Thanks in advance!!!