Those bolts are for a rear rack. Just keep them in place if you dont use a rack so that water doesnt get into the tubes.
That design is called a wishbone seatstay. Seatstays are the 2 tubes that connect the top of the seat tube to the rear dropouts. A modification to that is to have a single tube connect with the top of the seat tube, like you have. It looks like a wishbone.
Also, you do not need to get cantilever brakes. You can keep the brakes you have and buy
TRAVEL AGENTS which will allow the V brakes to work with road brake levers. Another option would be to purchase
miniV brakes as they have the same short pull ratio as road brake levers.
I mention these two options because they allow you to either use the brakes you have and/or avoid cantilevers.
If you are deadset on cantilever brakes, then get this
CABLE STOP or something like it. It connects to your seatpost binder bolt.