Re: simply editing startpos for Sassanids as a mod
Welcome to TWC, fuzzyion. This sounds very frustrating.
While I haven't modded the startpos for Attila, when I did a bit of startpos editing for Empire Total War, I found husserlTW's guide helpful. Even if some details are different for Attila, this might help you to make sense of how startpos editing works.
This guide to adding victory conditions might help, even though you're simply trying to edit them, rather than add victory conditions for a new faction.
Good luck!