yogurt, boiled milk?
Probably. If the milk boiled, it might have killed off too much bacteria to culture properly. I base this theory on the fact that ultra-pasteurized milk is not recommended for making yogurt or cheese. UP milk has been heated to a high temperature to extend its shelf life. It can result in a very runny yogurt.
I would start over. Use that boiled milk for something else, like custard or ice cream.
yes! I've done this many times. it will work fine, assuming you add your culture AFTER it boils when it cools to the correct temp.