HYPP N/H horses aren't the same as carriers. They are afflicted invididuals suffering from a genetic disease that can, in the worst case scenario, be fatal. N/H horses can become symptomatic at any time, even if they've gone 20 years without an attack, which is why many people consider them to be unsafe for riding horses--you sure don't want your mount collapsing out from under you in the middle of a trail ride. Generally speaking, N/H horses are less severely affected than H/H horses, but there are no absolutes, and they can still have a very bad form of the disease. There are dietary and exercise methods for reducing some of the risk, but again, there are no absolutes and no guarantees. HYPP can be miserable for a horse.
Breeding an N/H mare to an N/N stallion means 50% N/H foals and 50% N/N foals; breeding her to an N/H stallion means 50% N/H, 25% N/N, and 25% H/H.