Coming at this from a payroll perspective -- I think they have constructive receipt issues on ALL the income and the IRS will not like this at all. This was not a mistake, but a changing of their minds. From a payroll/tax perspective this is a nightmare approach especially if they have also already submitted FIT/FUTA/FICA and then any state consequences.
I don't see where this could be claimed a mistake in fact. They got the compensation, they have deferral elections in place, etc. This is just bad planning on their parts.
If I were their accountant, while it does cost them more in taxes, I would suggest leaving it as is and doing a cash investment of the net pay back to the company. Because might just cost more than they had in taxes to fix all the different parts (of which the 401k deductions are just one). Add in loan repayments and it gets even nastier.