My 1st Mobile app using Drupal 7 as a backend

Good Day,

Noob question here. I am interested in developing a simple mobile app for the iPhone and Android(clients) using Drupal 7 as a backend. I plan to use Titanium.

Mobile App Functionality:

1. Register/Login Users
2. Post Content (Title, Message, Geolocation, Images and Video upload)
3. Show other user submitted content
4. Update/Delete user submitted content
5. Message Inbox (maybe)
6. Simple User profile (maybe)
7. Push Notifications

Drupal End

1. To show content (Title, Message, Geolocation, Images and Video) posted from mobile app.
2. No user registration

Before starting, I am seeking some information and guidance.

My Questions

1. Account creation
   A. Will I need a separate database for this info? At this point, I am not allowing user creation from Drupal. Will this be a problem?
   B. How to authenticate registered users? Oauth or are there other means?

2. Content
   A. What is the best way to import data from to Drupal and export to client? I have read some use JSON and some prefer XML. What are the major PROS and CONS?

That is it for now.

Thank you very much for your patience.

Mark