Hi Aleithaki,
Thank you for this detailed post as I can see where is the problem right away
When you open Saves folder, you should see several folders named the way you named your save files in SV. Open one of them and you'll see several files including those you mentioned that you extracted in your Documents/Electronic Arts/TheSims3/Saves. You should extract
entire folder with those files in it - folder is named Union Cove Start and should be amont other saved files folders, not just files. Then, you will see the UnionCoveStart save in your main menu and will be able to start the game with it.
In your Saves folder there should be ONLY folders, like this:
Of course, they will have the names of your saved games.
I hope this will help.
New updated version of Union Cove should be out soon so you may want to wait for that one - it will not change much, but some lots will be more functional - fixed spiral stairs, one farm will have better routing for horses, a street light won't sit on driveway, bars will have less professional bars so less NPC 's will be spawned, etc.