You misunderstand slightly, decompression IS necessary; I've just done it for you. All you literally have to do is put the file you got off your xbox360 into the editor via the open button. That error indicates the decompression toolkit hasn't worked, if it said it's crashed then something is wrong with the save, otherwise I don't know what to tell you. I haven't coded in checks for that yet, what I did code in however is the check for the right file you put in, so if you try and put anything in other than an xcom enemy within gamesave it will tell you, I check it's a CON and I check its the right game, you can't bypass that without trying.
As far as I know it should all work right up to the decompression, try running as admin (it's safe, promise.
) and keeping the saves in an area you know you have write access, desktop is fine.
I don't think you can mod the weapon damage, thats not something they put in the saves, more of a disc data thing since it never changes excluding when a title update changes it.
I'm moving the weapon fragments up in priority, I too am constantly running low on them, I'll make sure it's in the next release, haven't actually worked on it today, can't be bothered. I'd love to open source this right now and get some other chaps on it, saves me doing all the work.