It is not entirely clear whether adding a member of a controlled group to an existing plan that is already sponsored by another member of the controlled group counts as an amendment or a plan adoption. If it is a plan adoption, then you should be able to do it retroactively under the new 401(b)(2) as added by the SECURE Act. If it is not a plan adoption, then it is an amendment, and it should be able to be adopted retroactively under the rules of 1.401(a)(4)-11(g) for purposes of correcting a failed coverage test. Either way I think you should be able to do it.
About the ABT, read your plan document carefully. Even with the failsafe language, you might not be precluded from satisfying the coverage test using the ABT. If the plan allows discretionary profit sharing contributions, then you may not even need to use a QNEC.