Not for now, but they should. The problem is that you need to take your customer to another site and that screws all the transparency (Doing everything in just one site) and simplicity that you have right now.
Indeed. Its a shame though, at least not to offer it as an option, as it places a lot more liability on the merchant. At least as an option you can profile your customers first and choose who to use it on.
I've seen some pretty scary numbers on the effect of 3-D Secure payments to conversion rates. I guess it's really hard to do The Right Thing™ on this matter.
Numbers for good or worst? They help A LOT fighting fraud. I think a good solution could be implementing it on Stripe.js.
The latter. It's probably a combination of breaking the UX by redirecting the user to his/her banks' website and the crappy forms the banks made for this.