Peace Commissioner

1. The declaration does not have to be written in the presence of the commissioner. It very rarely is.
2. The usual form is that the declaration is prepared and brought to the commissioner and the declarant signs it in the presence of the Commissioner and avers (verbally) that it is true. The Commissioner does not read the declaration and does not know (or care) what is in it, or what it relates to. His role is simply to confirm that the declaration was made by the declarant.
3. Strictly speaking, it's not necessary for the declarant event to sigh the declaration in the presence of the commissioner. A declarant could bring a pre-signed declaration to the Commissioner, point to it and say something like "that is my name and handwriting and I say that the contents are true". But as the declarant has to appear in person before the commissioner anyway, there is no particular reason not to sign in the commissioner's presence.
4. Yes, a sworn declaration can be defamatory. But declarations are usually made in the context of court proceedings and so are protected by privilege.
5. You wouldn't normally make a declaration saying that "X was to give something to Y but Y has not confirmed receipt". The person who can give evidence that Y did not receive the thing is Y, so you would want Y to make a declaration saying that he never receive the thing. Somebody else saying that Y never received the thing is not the best evidence.

You can say "X said that he would give it to Y"; that's evidence as to, e.g., the intentions of the person who gave whatever it was to X, the commitments X made, the purpose of the whole transaction, etc, all of which might be relevant to the dispute at issue. But if you want to prove that Y never got it, you need a witness who can testify to that from their own knowledge - ideally Y but it could be e.g. Z who got it instead of Y and still has it, or W who say X give it to some third party, or destroy it, or whatever. A, who gave the item to X to give to Y, is unlikely to be in a position to give direct evidence, from his own knowlege, that Y never got it.

No, the commissioner does not keep copies of the instruments declared before him.