I think it is pretty clear that it is not deductible in 2017 if not deposited by the due date of the return. But I know of a lot of accountants that would not want to be bothered amending a return and would take their chances, and I don't see it as the plan's (or TPA's) problem as far as that goes. If there was a legitimate election to defer that much, then I think you have a legit deferral and legit late deposit issue. Amending the return to remove the deferral (and not making it up) creates a train wreck of amending the 5500 too, and in theory a failure to implement a deferral election, although in the case of a sole prop that issue is somewhat blurred.