Re: ETW unpacker
I am not sure what you mean by an unpacker. Do you mean a tool which can be used to make modded pack files? The Pack File Manager can do that for Empire Total War, the most recent version is available from the Tools, Tutorials and Resources area of the Mod Workshop for Total War: Attila, here. (You do not need to own a copy of Attila to use it). Some older versions of Pack File Manager are also available, for example I often use a combination of PFM 1.5.9 (available from the Tools, Tutorials and Resources area of the Mod Workshop for Napoleon Total War) and PFM 1.5.3 (a feature of husserlTW's EasyDB tool, available via husserlTW's My Modding Work page.)
If you want to be able to modify a copy of the startpos.esf, then you would need an ESF Editor - various versions are available from the Tools, Tutorials and Resources area of the Mod Workshop for Empire Total War.