Regarding the witnessing of signatures, can one of the Primary Beneficiaries witness the signature of the Trustee?
The only restrictions on who can be a witness to sign a trust deed are that the witness is not under a legal disability - e.g., being under 18 years of age would prevent the person being a witness.
However, each State and Territory has different rules about when deeds will be validly executed. Some of these include the requirement that the deed be witnessed by an independent person. Therefore, it may be worthwhile having a person unrelated to the trust arrangement witness the signature.