I love Leonard! It's such an intelligent but cuddly-sounding name. I think it's due for a revivial, too.
-I pronounce it LEN-nard, and it's the only pronunciation I like.
-I think Leonard is safe as far as spelling and pronuncation are concerned. It's a well-established name with some famous bearers, like Leonard Nimoy and Leonard Cohen. There are no guarantees--some people just can't spell--but I think misspellings would be rare rather than the norm.
-More common? No, or at least, not yet. I think it will go up the charts just because I think it could appeal to the hipster set. Leonard fits in really well with names like Hugo, Rufus, Hazel, and Olive. I don't think it will be really popular, though.
-Absolutely! I'd use Leo if I had a Leonard. It may not be pronounced, but it's right in there in the name. I also like Len, and the nice thing about Leonard, as opposed to something like Leopold, is that it gives the kid nickname options.