Personally - I'd think it was nice if you learned how to sign "Welcome" in response to her signing thank you - and then see her reaction.
Regarding Pen and paper - one idea is just leaving a note pad with a pencil on the order counter in easy reach. I never am offended if someone offers me pen and paper, provided it's a " would you mind writing that down so I make sure I get your order right" - and it's not shoved in my face !
If you're a local shop with a fairly straightforward menu - you might even consider printing up a "check mark-able" menu that customers could take home, or check at the counter - not only does it make you more hh/d and speech impaired friendly - it might encourage people to try new things as well !
Just a thought
