Assume a Participant should have entered the Plan on March 1, but was not given the opportunity to defer until much later. If the Participant elected to make no deferrals when they were given the opportunity is there still a need to make a corrective contribution reflecting the "missed opportunity" deferrals? No matching contributions were made under the plan for that plan year.

Yes
Then no need to make a corrective match

I don't know what "much later" means, but there is a de minimus rule where a correction is permitted if the employee can increase deferrals for the last 9 months of the year. In that case, corrective contributions are not necessary. (Rev. Proc 2008-50, App. B, Section 2.02(1)(a)(ii)(F).)

They were excluded until this week! I wasn't sure how to apply that example since we're not dealing with a 12-month period as they should have entered the plan on March 1, not January 1. By allowing them to defer now we would be able to offer only 6-1/2 months to defer. Would 6-1/2 months of deferrals be satisfactory if the plan allows deferrals of 100% of comp? Who knows?!