Yes I expect that every premise will require some explanation and defense.
For premise (1), I think we can demonstrate it in two steps.
First, we show that moral judgments always presuppose some kind of moral norm. For example, the statement: "You shouldn't go faster than 45mph on this road" presupposes the norm of the speed limit. This norm is not absolute, but it is a norm.
Second, we show that relative norms always depend upon absolute norms. A relative norm is something that can always be evaluated from a higher level. For example, human laws have some normative force, but we may always ask: "Is this law just? Is this law good?" In doing so, we are appealing to some higher moral norm by which human laws can be judged. At some point we arrive at a norm which cannot be measured by anything else. This norm is absolute.
For premise (2) we should point out that norms always arise in personal contexts. In other words, a norm is always imposed upon us by a person. We have obligations to persons and are held accountable to persons. If I say: "You shouldn't go fast than 45mph on this road," you might respond by saying "Says who?" By whose authority am I not allowed to travel faster than 45mph? Norms are always imposed by authoritative persons.
From there we can see how an absolute norm cannot be imposed by a non-absolute person. It must be imposed by an absolute person.