Arg, lazy morning… I didn’t get it right Once your update will be accepted, you’ll receive a mail. But your not customer.
My minimalist WordPress theme got accepted without problem… and it’s my first one. You don’t have to worry.
For the additional services, I think you can do whatever you want to help your customer… Then you’ll have to see with if they want to pay for your service, but you have to manage this out of ThemeForest for the fee.
To have an update it depend on the autor so you can ask him if he will do it. But I don’t think you’ll receive a mail for the update, you’ll need to check it out yourself, or follow the autor on twitter.
And to use the update properly you can use the parent / child theme method. But I’m not sure we can do it with all the theme, need confirmation
Maybe someone can do a WordPress version?