For selecting your homecity in the Scenario Builder, the only thing I can think of is to look at the player options on one of the menus and see if there is a box there to select the homecity for Player 1. Other than that I'm not sure.
Secondly, I would not try to load the skirmish game you mentioned again. It would probably also be worth deleting the save game from My Documents\My Games\Age of Empires 3\Savegame\YourSkirmish.age3sav
Thirdly, to return your homecity to level 76:
1. Make a copy of the following file and place it somewhere else on your computer (your desktop may be a good place) - My Documents\My Games\Age of Empires 3\Savegame\YourHomeCityName.xml
2. Open the original file (not the copy) using Wordpad - My Documents\My Games\Age of Empires 3\Savegame\YourHomeCityName.xml
3. Search for and then set the following values:
...76 'home city level2050000 'experience must match home city level to be accepted by game76 'number of available cards to buy0.63747576 'number of customisation points
*You should only need to change the homecity level and xp values (you most likely will not need to change the skillpoint, xppercentage or numpropunlocksearned lines).This thread
shows how much experience you need for each homecity level. I put in the correct value for level 76 for you (if the value you have is higher than 2050000, then keep your value).
4. Finally, restart the game and check your homecity.
If you feel you've messed up the homecity file, you can put the backup copy you made earlier back into the My Documents folder.
Hope this helps :)
[Edited on 7/19/2006]