You can NOT reset your city to level zero, if you have logged into FB (etc). If you reinstall the game or reset its data, you can start again at level zero, but if you log back in once that feature unlocks, you'll have to load your old saved city to access your friends/neighbors again.
Level 15 is not so bad. But by the time you unlock nearly everything, around level 30 or so, there will be so much other stuff in the trade HQ it will become almost useless much of the time, considering the way that feature is currently set up to 'work'.
The main thing is to be prepared BEFORE each level up. You can make as much coin in a day from producing materials consistently and selling them as you can from upgrades, and you lose less value using materials for keys than you would with more expensive stuff unlocked too.