"If an employer were going to defer, that deferral would happen on the payroll tax return"

And that wording also warns the employer that any amounts already paid in through the 941 deposit process cannot be refunded for that period where you would have wanted to keep the money (defer deposit), as an afterthought process (when filing the form 941 or 944).

" "However, you can't defer amounts that you have already paid."

Does that mean they'll use any refund on the 1040 to offset the deferral?"

It means the other way around. Think of this as a step towards parity for the self-employed to get to the same place as employers and employees for these deferrals.

"30. Is a self-employed individual who defers 50 percent of the Social Security tax on net earnings from self-employment income, or a household employer that defers the employer's share of Social Security tax under section 2302 of the CARES Act, eligible for a refund of the deferred amount of tax at the time the taxpayer files its Form 1040, Individual Tax Return? (added July 30, 2020)

Generally, no. A taxpayer who has deferred his or her payment of the employer's share of Social Security tax or 50% of the Social Security tax on net earnings from self-employment under section 2302 of the CARES Act is not eligible for a refund due to the deferral because the deferral amount is a deferral of payment, not a deferral of liability. Therefore, the deferral itself does not result in an overpayment of taxes reported on Form 1040. However, if a household employer is eligible for advanceable paid leave credits under the FFCRA and reports those credits on Schedule H, Form 1040, the taxpayer may receive a refund of the paid leave credits even while deferring the employer's share of Social Security tax. This does not apply to credits for sick leave and family leave equivalent amounts for self-employed individuals.

Self-employed individuals and household employers should consider deferrals under section 2302 of the CARES Act in determining their estimated tax payments and any income tax withholding from wages and other sources of income. Publication 505, Tax Withholding and Estimated Tax for use in 2020 provides more details on determining these amounts."