2. Its possible that the entire galaxy knew that the stormtroopers were clones, so Han and Luke wouldn't have had any reason to be surprised.
3. I think the rationale behind NOT redoing all the voices is that, even with cloning, individuals are going to have different things happen during their lives that will cause their voices to be different.
Also, cloning is not 100% perfect.
I don't know if you've seen Stargate SG-1, but an example of the troubles with the cloning process are evident in the Asgard. Their species is (or at least was) dying because their cloning techniques caused DNA breakdowns.