Just to add a note regarding this issue of merging plans (while sympathizing with everyone's frustration in M&A situations when the M&A attorneys ignore retirement plan implications): when you are trying to merge two 401(k) plans mid-year, you have to consider that the merger acts as an amendment to the plans. So, if you have a SH nonelective plan and a SH match plan and want to merge them mid-year, you must realize that the impact of that is to change one of the plans mid-year from one type of SH to another type of SH, which is expressly prohibited under Notice 2016-16. Similarly, if you have a non-SH plan and a SH plan and you merge them mid-year, either the non-SH plan is being amended mid-year to add a SH provision OR the SH plan is being merged mid-year to terminate a SH provision. Either way, you must conform to the rules of Notice 2016-16, as augmented by the SECURE ability to do a mid-year adoption of a SH plan under certain circumstances. So, beware of mid-year mergers. Better to wait until year end, freeze the acquired plan as of year-end, have the acquired employees enter the buyer's plan as of the beginning of the year, and merge the plans either concurrently or ASAP after that takes place.
Also, in re the coverage issue raised, remember that you likely have the transition period under 410(b) unless you amend the plans in the meantime.