@tony Since, as pointed out by Joonas, the comment chain is getting long, I follow his kind invitation to address the final question you raised in your comment in https://latin.stackexchange.com/questions/15055/grammatical-structure-of-obsidibus-imperatis-centum-hos-haeduis-custodiendos-tr , i.e. “ Whether the gerundive is optional or not, do you agree that "hos custodiendos" can be treated as a single, discrete unit--the accusative-plural direct-object?”.
As a preliminary remark, let me point out that it is not the case that what is accusative is always part of the direct object. For examp…