I think your question about whether there is a distributable event is addressed in the definition of a severance from employment - see Code section 401(k)(2)(B)(i)(1).
This discussion is from IRS Notice 2002-4:
Under Code § 401(k)(2)(B)(i)(I), as amended by § 646 of EGTRRA, amounts attributable to elective contributions may be distributed upon the employee’s severance from employment with the employer maintaining the plan. For this purpose, the employer includes all corporations and other entities treated as the same employer under Code § 414(b), (c), (m), or (o). An employee does not have a severance from employment if, in connection with a change of employment, the employee’s new employer maintains the section 401(k) plan with respect to the employee (for example, by assuming sponsorship of the plan or by accepting a transfer of plan assets and liabilities (within the meaning of Code § 414(l)) with respect to the employee). Thus, for example, if all employees of a controlled group of corporations (within the meaning of § 414(b)) are covered by a section 401(k) plan and a transaction occurs such that one subsidiary corporation in the group is no longer aggregated with other members in the group under § 414(b), (c), (m), or (o), and in connection with the transaction no assets are transferred from the section 401(k) plan to a plan maintained by the former subsidiary corporation, then, participants in the section 401(k) plan who continue employment with the subsidiary corporation will have a severance from employment with the employer maintaining the section 401(k) plan and may receive a distribution of amounts attributable to elective contributions from that plan. However, if the subsidiary corporation maintained a section 401(k) plan for its employees before the transaction and continues to maintain the section 401(k) plan following the transaction, the employees who continue employment with the subsidiary do not have a severance from employment with the employer maintaining the plan.
As you stated, the Plan document doesn't mandate that there be a spin-off/transfer of assets to a new plan maintained by the employer leaving the controlled group. However, the Plan document includes the severance from employment definition. If the employers agree on a spin-off/transfer, then it is not a distributable event when the subsidiary (?) leaves the controlled group. Otherwise it is.