Actionscript is easy, you could try it out.

Yes, it definitely is worth learning Actionscript 3.0 for AIR apps.

Actionscript 3.0 is very easy to learn and you can build good games with it.

Javascript and Actionscript are very similar. You can learn Javascript and then just pick the differences in a few days (static types, Class, etc.).

I would go with Actionscript. In my opinion, AIR's support for JavaScript is mainly to make adopting AIR more feasible for people with a background in web development with JavaScript. Using Actionscript will make your life easier. Sadly, live debugging JavaScript is still a pain compared to other languages. (Although, if you decide to go that way, WebStorm in conjunction with the Chrome DevTools is fairly decent and probably as good as it gets.) Whereas, if you use Actionscript, you got FlashDevelop, an easy to use (and free) IDE that makes writing and debugging Actionscript really pleasant.

I would go with actionscript as well, it is an easier/quicker learning process if you already know PHP. Plus easier to debug. I kept an old version of the eclipse plugin for flex development that I could use for debugging in a pinch, I don't know that it is easy to find these days.

Out of curiosity why AIR apps? There is less support for AIR (none on newer mobile devices) If I were doing new development for in house tools, AIR is fine, but for public web sites I would lean more towards jQuery or HTML5 development since both tend to be more portable these days.

I think javascript is better for air apps.Javascript is a easy language and you can easily implement the code.Many javascripts tutorials are vailable on the internet.