Developing Mobile apps with AS3

There's many ways to skin a cat, most of the answers are entirely based on what you want to do.

1. Depends on the game. In general the Flex (spark+) components are mobile optimized but there are Stage3D (FeathersUI, etc) GPU-drive elements that run at near native-like speed. The components are not nearly as easy to work with (although the codebase is trying to be unified for ease) and they do follow some common component conventions. So based on what you want to do, possibly Flex, possibly Stage3D.

2. All my apps have 100% custom UIs. I use almost absolutely no components or anything premade that I don't wildly extend. That's the power of Flash. 100% custom UIs are much easier to develop.

3. They run on completely different levels. Flex uses a different language (MXML) with some hand holding and different approaches to developing apps. Components are used heavily and often skinned quite a bit to get a custom look. They're easy to maintain and scale. AS3 projects cannot use the Flex layer so you'll be utilizing components designed for Flash Pro versus Flex. Many are similar. The approach to how the app is built can be more flexible but require a lot more hand coding. You use AS3 to do it all and no MXML. People from Flash Pro should probably migrate and try an AS3 Mobile project, learn the basics and then take a look at the MXML language.

4. Advantages are a highly reusable code-base between different devices. If developed properly much of the AIR framework is unified so when deployed it can be ported very easily to iOS, Android, BBTab, Desktop, etc. With Adobe Native Extensions we can also tap into certain portions of native coding that the AIR framework doesn't yet support which is IMHO one of the biggest benefits. The limits are you're cross-compiling to arm/etc and no matter what, even with Stage3D, to get native performance there is no faster way to develop basic applications than native. You have no limits or require ANEs to overcome basic obstacles and everything "just runs great". It can take a lot of time to get even close to that performance without native.

5. Yes Flex has script tags and it's weaved into the overall usage of Flex quite a bit. You'll end up getting very used to heavily mixing MXML and AS3.

6. Only if you code poorly but hopefully your app will be sniped from the store if this is found to be true. Same rules as native apps. Don't do silly things like expect the OS is impervious and save raw credit card data on the device, etc etc. No advantage either way from a security standpoint.

7. Of course. As well as tutorials online. Google and ye shall find.

8. Entirely based on devices and the ANEs that can be developed. You get the typical basics, write/read storage in your sandbox, use the internet, camera, microphone, sensors, etc. Android requires the user to authorize it, iOS doesn't. You should always warn a user about your intentions with their devices.