Thanks for your help and info.  I read those articles that you listed. 

I think what I'm "hung up" on is what that looks like in terms of tax forms and such for the following year IF it's not corrected in time (also, I've never had a client that's had to fly this close to the sun for the deadline).  I get the excess deferrals would be taxable as "excess salary deferrals" in 2020...easy.  And I understand what would happen if the issue was corrected in time by doing the "1099-R code P in 2021" thing.  So if it's not corrected in time, is that something that's denoted differently on a 2021 1099-R?  I imagine if it's not done in time it's reportable in 2021, along with the earnings that would've already been reportable in 2021?  Like...Are there special codes that get recorded on that 2021 1099-R or other additional information that gets included so that the IRS knows my client isn't subject to the "excess tax" anymore???

Please forgive me for my lack of knowledge surrounding this issue.  I've just never run into this situation before and am trying to learn how best to advise my client and prepare the tax returns properly moving forward.  Thank you again for your input!