5. No statement ascribing a mental predicate can be derived from any set of purely physical descriptions.
Meh. Hume's Law doesn't really apply to that statement. That's the same as the Chinese Room argument against strong ai. It is more wishful thinking than a valid thesis. It is very easy for a collection to have properties that its individual elements do not. A simple example is that a set of objects can be assigned a number corresponding to the number of objects in the set, whereas that property isn't meaningful for a single object. Once the collective property is defined, it is very easy to logically derive it from the collection of individual objects. So a particular mental state can be assigned to a particular set of neurons firing and, assuming that you have a good neurological model of the mind, you can therefore derive mental states from physical descriptions in the same way you derive results in calculus once you have defined concepts like limits.
To come back to the point of the thread, attempting to save humanity is meaningful even if it may ultimately be unsuccessful.