I've made a white label app before. I can answer most of your questions very briefly:
a) One database that supports multiple apps.
b) One API that supports multiple apps.
c) This is a big decision. You can use MonoTouch, Cordova/PhoneGap, appcelerator, rubymotion, Adobe Air, hybrid web app, etc, if you wish to make a cross platform codebase. Or you can make separate code bases for both iOS and Android. In our case we made separate code bases for iOS and Android in order to make the best app possible. We had one developer for Android, one for iOS.
d) In your scenario, if 100 people sign up for your app and you need to push an update to the store, you have to upload the app 200 times (100 for iOS, 100 for Android). I'm more experienced with iOS, and I can tell you if you want to automate it, it has at some times been possible, but very hacky (unreliable). In my experience, we just uploaded the apps manually. It sucks but that's how it is. I wouldn't be surprised if Android is easier to automate.