A port of SFWMill to Haxe is not yet a complete implementation of SWF format, but it is close enough. This would allow you to create an online standalone assets compiler, with no back-end requirement. It is in Haxe language, but Haxe is very similar to AS in many aspects.
FLA is a proprietary format, meaning that even if you knew it's structure, reversing it may cause legal problems. Besides, why would you want that for yourself? I'd imagine an old proprietary format having all sorts of patches, irregularities of kinds etc... just go with something that is easy for you to edit.
AS2 is not being worked on for a few years now. It's not officially discontinued, but no bugs are accepted for that language and runtimes any more. When you write in AS2, you are pretty much speaking Latin to Italians.
Regarding online compilation of ActionScript: There is an online compiler written by hurlant. It may not always act the same as Adobe compilers do.
There are other tools for compiling to AVM bytecode, specifically, http://haxe.org/com/libs/format/abc and http://www.as3commons.org/as3-commons-bytecode/index.html.