Re: Never heard or seen this
This part makes no sense: "Once the payroll was ran in Jan. we would need for quick books to adjust the bonus checks & payroll tax back to Dec 31 2018."
You never Backdate. As noted, even if you Qualify for this, as an Accrual entity, you accrue the amount owed, and then Paid out on 2019 paychecks are part of 2019 Banking, never Backdated afterwards. You never Change Payroll dates as you seem to be proposing.
In the US payroll taxes are owed because the Money happened, and that is based on "Constructive Receipt" which is the Pay Date, and that never gets edited, later.
I would find a different CPA or accountant, because you are not getting good guidance from this person.