1. They will, unless you wall them in. See #2.
2. You can wall in the room with a constructed wall with supports holding a natural wall a z-level above, but make sure to also dig out the area only one z-level below the constructed wall so the natural one doesn't just break the other and replace it. That's a base design I use in most of my vaults.
3. If you wish, you can keep them, but that's your choice entirely. Whenever adventurers activate traps is aii in the init file.
4. I think that if it stays in barrels it shouldn't rot. Think being the key word here.
Not sure on #5 entirely, so going to skip it. Someone else probably knows more on this.
6. All dwarf-made armor and clothing is made for dwarves, which are smaller than humans, landing you a lot of unusable equipment. Elves can use it, if it helps.
7. You could make yourself a live magma pumpstack complete with reactor ready to start flooding things as you reactivate the reactor itself (I found it useful to have a windmill farm, which by lever connected to gear assembly is connected to the main reactor, which is then via lever connected to gear assembly connected to the stack. You're going to have to guess whenever it's all active from the looks though.) Access to all the caverns should be avoided, unless you plan to do it in the vault manner described above, since you're going to have a funfest with it's various critters once you come back as an adventurer... Rest's up to you, it's your fort after all.
8. Varies whenever you want to achieve Fun or not. Abandoning will leave a bunch of critters from the areas you have access to, succumbing will leave in goblins, and inviting the clowns will cause them to stay for a while. Note that if you don't get to the toothpick storage before meeting a clown you may be in for a very, very Fun time.
Hope it helps.