I think it would be best if Shinichi tells Ran the truth about everything - the truth about Conan's identity, the truth about his predicaments, the truth about why he couldn't have told her sooner. But you're idea is plausibly convincing. Ran isn't the smartest bird in the bush, and her gullibility doesn't do her any justice either. I'm pretty sure Ran wouldn't refute any of Shinichi's testimonies about his disappearance; she would rather be on cloud-nine once he returns (if he returns).
But if that were to be the case, wouldn't she want to see Conan again? Would she have the chance to say goodbye to him once Shinichi returns? It's pretty predictable, if not obvious, that Shinichi will eventually tell Ran that he shrunk into the body of a 7-year old boy named Conan. Explaining to Ran that he and Conan are two separate people would only stirrup more trouble and raise even more suspicion as time goes on; therefore, Shinichi will have to tell her sooner or later.