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S corp/C corp doesn't matter for this purpose. Deferrals have to come out of a paycheck(s) by 12/31. Once that happens you have the timing of deposit issues (e.g. 7 business days for a small plan) but that is a different matter.

Agreed. Brokerage firms can code all they want but that info doesn't go anywhere but their own systems; it's not automatically reported to anyone (IRS). If the IRS should ever review your records, under audit or otherwise, yes they will see the coding from the brokerage firm that they are 2017 contributions, but just keep your correspondence and your tax returns and show when you really deducted the contributions and that should take care of it. This all means that you pretty much have to deduct the extra in 2018 though, it could be a 401(k) contribution or an employer profit sharing contribution.* *Off topic but just for the record, you really can't scratch your head on Sep 11, 2018 and decide to take a $24,000 401(k) contribution for 2017. That election should have been made by 12/31/17 (e.g. on a paper form). "Everybody does it" and it's apparently not enforced at all.