I just want to go in a small tangent here, but I imagine that if Cesar knew he was going to die and had to adopt someone to continue his rule, he probably wouldn't pick his sickly nephew with no experience, no matter how bright he might be. So I think it's very likely that Cesar put him in his will since he is a blood relative and didn't really expected to die soon anyways, but didn't mean to have him as his sole heir forever.

That's a good point. Did Julius not have a mental disorder which caused him to lack fear? It's a good for Octavian that he got close to his great uncle while he could.