How do you email the restaurant for reservations? The web site isn't very helpful.
Sorry, I can't help on this one, we were part of a small group and the reservation had been made for us.
Alternatively, perhaps you can get yur hotel/airbnb to call them to help you make a reservation. They do accept walk ins too, but if you're intending to go at the peak of lunch and dinner and especially if it's a big group, probably reservation would be better :)
It seems to be a family run restaurant so I would advise calling them.
It's a small place for locals and tourists. Your best bet would probably be calling the day of or maybe the day before from your hotel.
Get quick answers from Halldorskaffi staff and past visitors.