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You you can definitely make a concrete message type from a <Message>Ptr type. Because the original type is a pointer you'll need to copy the entire message to do this, fine for small geometry messages but it will have a performance penalty for larger point cloud or image message types. You can use the copy constructor to create a local concrete instance of a message which is the same as a given <Message>Ptr type. This is the same as if if was a regular C++ pointer.

void myFunction(geometry_msgs::PosePtr postMsg)
{
    geometry_msgs::Pose concretePose = *poseMsg;

    // ... do something with concretePose 
}

Regarding your second point the geometry_msgs::PosePtr is actually a boost::shared_ptr<geometry_msgs::Pose> not a simple C++ pointer. A Boost shared pointer is a smart pointer container that ensures memory will always deleted when no more references to the object exist. You can read more about these here.

Hope this helps.