A week or two into 12/12 shouldn't be a problem taking a couple clones. I took 3 clones from my last grow at 2 weeks into flowering and they rooted and are growing fine. Although I must add it has taken them a bit longer to come around than the ones taken in veg did, and I'd suspect that is because they have to re-veg first.

You can re-veg a plant. I don't know much on that subject as I've never done it, but I know it can take a couple months, and that when you harvest, you want to let enough leaves/bud on the plant so it has a fighting chance. I'm sure someone else can go more in depth with this.