Disclaimer of interest in IRA accounts passing to see-through trusts

I have a question about the proper drafting of documents and procedures to ensure that eligibility for the 10-year rule for inherited IRAs is maintained.

Facts:
• IRA accounts are split between two see-through accumulation trusts for benefit of sons at IRA Owner’s death.
• One or both of sons, at Owner’s death, may want to disclaim his interest in order to allow it to pass to his children, who would be sole, equal beneficiaries of the trust if Dad disclaims.
• The beneficiary designation forms provided to the custodian of the IRA accounts will identify the sons’ respective trusts (established under the terms of a revocable living trust set up by IRA Owner) as 50%-50% primary beneficiaries.

Question:
• What is the proper way to handle a potential future disclaimer by a son?
• If a son disclaims, the IRA accounts will still be held in trust, albeit for the benefit of his children (Owner’s grandchildren). Must the beneficiary designation form name the grandchildren as the ultimate contingent beneficiaries if the son disclaims? (The same trust, but with different primary beneficiaries, will receive the distributions from the IRAs.)
• Would the disclaimer take place solely at the trust level, with the son providing a written disclaimer to the Trustee?
• Must the son disclaim in a document to the Trustee, with the Trustee then notifying the custodian of the waiver and providing the names of the ultimate beneficiaries, i.e., the grandkids?

In short, in practice, how does one accomplish the stated objective (avoiding a disastrous result like in PLR 200846003, where the children’s disclaimer to benefit Mom’s purported QTIP resulted in the deceased IRA owner’s estate being treated as the beneficiary)? Thanks in advance for any thoughts.