starting a 2017 safe harbor - missed deadline

Hoping for any comments as to whether this would fly or not:

We have a small employer looking to start a safe harbor 401k for 2017.  Its 11/10/17 so the deadline for 2017 has long passed.

However, could they start one 12/15/217 and have the plan year run from 12/15/17 - 12/14/18.  Have a short plan year 12/15/18 - 12/31/18.  Then, go to calendar year for 2019.  Their company fiscal year is calendar, so does that automatically sink this idea?

They have 3 employees, 2 of which are owners.  The owners would be receiving large bonuses which they could make in late December, 2017, with the owners deferring most of that to the 401k plan in late 2017.  They then have 11+ months to make 2018 deferrals.  do nothing for the 16 day SPyear, and then pick up on a calendar year basis for 2019.  The other employee will not be putting anything into the plan. given what is likely to be a high contribution rate for the 2 HCEs for 2017, a QNEC for the other employee would be pretty expensive and not desirable.

Thanks