1. The benefit, when only Happy Home Designer utilized them, was to add even more people to design houses for. The game itself offers a decent amount, but this expands things, and even unlocks items I think.
2. IIRC You have to cycle through 16 villagers before the internal memory erases a particular villager from its memory, before the cards were a thing I time traveled and knocked out 16 villagers before Cole was able to move back.
3. No, special characters stay special characters. However, you can still scan them with Wisp and receive items from them, and some special NPC's will camp out in your campground too.
4. No, when villagers move in via card they are not permanent, but if you have their card then you can always rescan it in after the cycle anyway.
5. You can buy cards secondhand, however some can be quite pricey. I've purchased a few cards just to complete my collection but overall I use the trade thread, it has helped saved me lots of money.
