Hello,
I've been working as a professional Flash Developer (and now manager of a team) for 6 years, I've had experience in the past with similar situations where we had to work back from a swf file due to the source fla being lost or corrupt.
We have experimented with a variety of swf decompilers where I work and narrowed them down to our preferred tools; some are better at maintaining visual elements, some are better at maintaining the integrity of code, instance, and linkage names. Using this experience I can try the tools we typically use to reproduce the fla from the swf you have in a format as close to the original fla file as possible, which I can then use to make the necessary modifications you need done.
Look forward to hearing from you!
Kind regards,
- Mike Boht