duckthing

Yes, for some reason (post count too low?) I can neither insert a link nor paste the address in, but the IRS has a page for "New Methods for Correcting Elective Deferral Errors". It includes a link to Rev Proc 2015-28 with the details. If you fall under the "early correction" criteria the correction would be a QNEC for 25% of the missed amount -- which, as you said, is easy to determine. Matching contributions with lost earnings are also due, and an end-of-year discretionary matching contribution, if any, would be calculated based on the deferrals they would have made had their election been followed correctly. Participant notice is also required, and the rev proc specifically addresses your question about participants having the option to increase deferrals; specifically, the notice must include language saying exactly that. [Edited: Sorry, for some reason I thought there was a model notice available as well. I must have imagined it. That's why I try not to rely on my memory!]

According to EOB the answer (with that assumption) is yes. Chapter 3, Section IV (don't remember the subsection off the top of my head... B maybe?). They do give a specific example about the situation where no contribution is made to one of two DC plans in a required aggregation group. I don't see this specifically addressed in the code anywhere, but this seems like the conservative approach at any rate. Hopefully this counts as enough 'research' to not be a "me too" post!

The language you quoted is only part of the picture. It says that your definition could be considered reasonable, but reasonableness is only the one criteria that must be met. The other two are that it "does not by design favor highly compensated employees" and that your reasonable definition also be nondiscriminatory (under the usual de minimis ratio test), so I think you're back where you started. I'm not an expert on this but unfortunately I don't think this is the quick fix you're hoping for.