Note there are
- Couple of features which are available only on Classic UI & Missing in Touch UI
- Adobe making major investments in Touch UI but not in classic UI
Following are the common usage now
- Touch SiteAdmin UI
- Touch DamAdmin UI
- Classic Authoring UI
Answers to your questions are
Make sure at least components working with Classic UI interface. I am sure engineering will find a way to reuse with less effort to move into touch ui for missing features.
Yes. Note deprecation is not declared yet.
Off course, look at basic principals at http://dev.day.com/docs/en/cq/current/touch-ui/touch-ui-concepts.html