Honestly, if I was getting a 'cool' phone for my mom, I would probably tell her to stick with a land line
But for real, for simplicity's sake for someone who is not on the technical side of things, I would go for the Instinct. My girlfriend has one and I've played around with it. Lots of features (for a basic phone), easy to use and pretty simple. What you see is what you get. It's fun to play around with for a while and pretty easy to get the hang of and learn how to use. Exactly what she wants in a phone. Everything you mentioned that your mom would want to do with the phone is pretty simple on the Instinct. To me, a WM phone is more for people who want to do a lot with/to their phone and don't have a problem spending the time and effort to do it. Which is exactly the reason I got a Touch over the Instinct. I get bored with the simple phones pretty quick anymore.
This opinion comes from someone who just recently popped their WM cherry. I got a Touch a few days ago and it's my first my WM phone. I love it. I'm a WM convert now. But I wouldn't even suggest a WM phone to my parents. They do pretty good to be able to check the voicemail on the simple phone they do have. It's odd to me... But the thought of either of them trying to work a WM phone is not something I want to witness, nor teach. And I know it's not hard... it's just too many options for them.
edit: Oh yeah, I also wanted to mention that the on-screen keyboard on the Instinct is big and seems pretty easy to hit the right keys. I don't know if that's a factor for your mom or not.