Severus dies in the book? Must have missed that part. ;-)
I love what you said about Albus Severus being a mix of Dumbledore and Snape. It seems so obvious now that you've pointed that out, but it was not intentional. It just made sense to me that Al would have an interest in potions (and possibly some natural talent, considering his grandmother). For the rest of it, I just kept a Slytherin mentality in mind. Interesting that in the end it worked out to be a good mix of his namesakes.
Hah, what am I saying, of course it was intentional! ;-)