I agree with your contention.
By "achievements is restoring the republic" do you mean in appearance only ?
Sure, nothing wrong with a partner/helper.
Augustus adopted Tiberius in 7AD, and it was he who was forced to adopt Germanicus, not Agrippa. So in forcing the adoption of Germanicus I think it more likely that Augustus just wanted a capable heir - certainly either a Claudian or a Julian, but either would do, as both Tiberius and Germanicus were Claudian. If at all possible, I agree, he would have like a capable son, but as it wasn't to be - was happy with a capable adopted one, even a Claudian.
Sure, I'm with you here.
I'm out of time, but you've some interesting specualtions regarding Gaius which I'll try to comment on later.