Q3. The standard way of doing this is to save as a normal game each time without saving as a scenario. Then when you continue you load as a normal saved game. There is nothing stopping you from saving as a scenario as well each time - just remember to load from the saved game.
Take note that you cannot use the events editor as each save game has its own stored copy embedded in the file. If you activate the events editor, it will overwrite the master copy with its own version. If you need to use the events editor, you have to start as a scenario, use the editor and then quit without saving.
At the end of it all, load the final saved game and save as a scenario.
Q1. When in scenario mode, there is a cheat button which allows you to get free city improvements in the screen where you choose what to build.
Take note that you cannot sell more than one improvement per turn so be careful what you give to the cities.