To fill in a few bits:
2. If she registers with the same credit card then it won’t be regarded as a new customer. This only means that nobody will get a referral bonus, but it’s worth mentioning.
3. You can in fact have a total of three domains and fifteen subdomains, even if the subs are all on the same domain. It’s just that you get 5 subs per domain into your pool, so if you buy another domain as add-on you get five subs more. But you can still use these under the same domain as the other fifteen.
5. Yes, you will be able to control all of them through the same panel.
7. don’t know.
Try out DreamHost with a free WebID – Prices, options