Hi @Alexy and welcome to playing, and to the forum :)
1. For save games, the game keeps track of which save is which according to its internal filenames in the save folders, not according to the name you give it. The name you give it from inside the game is just a convenience to you. If you want to make another copy of your save, you can either "Save As" from inside the game, or make a copy of the file from the save folder manually. When you are looking at your saves folder, the easiest way to identify the save game you want is to look at the time the file was last modified, which will be when you last saved that game.
2. Unfortunately yes, although whether you actually delete the household you create is up to you. When I'm starting a new game to play premades or a scenario, I like to pop a household into Create-a-Sim from my library to save time. Sometimes I leave them running around in the world as NPCs while I'm playing the other family I wanted to play.
3. Sounds like what you have is two different played households in the same save. (if you open Household Manager and see them both under "My Households," that's what you have.) For aging, yes, you're right that "Active Household Only" is what you want if you want the households to stay at the same age while not being played. I like to turn on NPC aging while playing one and only one household in a rotational game, so that the NPCs age at the same rate as one of my played households. If I want everyone to age at the same rate I make sure that I'm playing the households for about the same amount of time, like a week each or something. For story progression, yes, if it's turned off in Game Options it should be turned off for all households at all times. You can also fine tune it from Household Manager, just be aware that the settings are different for played households (My Households) vs. unplayed households (Other Households), that isn't very clear in the interface.
4. I'm not sure about this one, as I keep my aging settings so similar between saves. It never hurts to double check.
5. and 6. I don't have the new pack so hopefully someone else can help!
Have fun!
