This may sound like a strange tip, but try practicing Croatoan in a place where you can easily see the string. Doing that should tremendously help you hit the string (or, at least, it did for me).
The second catch where your thumb is pointed toward you is a completely different beast than the first one. I found it to take much more practice to get that one consistent. It may seem like you're not making any progress for a while, but keep at it, this is a trick you can get comfortable with. It just takes time.
Depending on how forcefully and what angle you pull your hands apart after the underpass-like move, you can create a completely different look and feel of the trick. It's amazing what types of variations can come from the basic movements.