I'm sorry that you lost your fish; that's never easy to go through.
The lifecycle of the marine ich parasite is pretty well established at this point, but that's a relatively recent thing. The problem with eliminating it entirely is that it's a pretty hardy parasite (as most are) and anything that could kill it would likely also kill the host as well - the fish, that is. In terms of treatment, then, we have to focus our efforts at eliminating the parasite during its weakest point of life - which happens to be when it's swimming around. Which it actually does twice in its life; we aim for the stage where it's swimming around looking for a new host fish, because at that point it's basically a baby parasite with a weaker protective structure. This is where copper comes in. Copper is poison and at the right levels, that poison will kill the parasite. At high enough concentrations, copper will also kill fish. So the goal is to get the copper high enough to kill the parasite, but low enough to not kill the fish. Because copper IS poison to fish, it's often important to give some of the more delicate fish a chance to acclimate to the presence of the copper in the water. Rather like how if you were to jump into a cold pool on a hot day, it's going to be more of a shock than if you were to slowly lower yourself into the water. Same thing here; if the fish is given time to get used to the copper, it can better handle it (to a point; copper can still kill them no matter how slow you introduce it).
In terms of the "white dots" on the fish; those are wounds. The "baby" parasite that's swimming around attaches to the fish and burrows in to get under the skin. This leaves behind a wound, which the fish's immune system attempts to heal via a scab. That scab is the white spot you see. This immune system response - and the extra energy toll it takes on the fish - is why it's important to ensure that the fish is eating well and in good health throughout treatment.
Once burrowed in, the parasite grows and matures. This process can take anywhere from 3-9 days studies have found. Once dug in, there's nothing that can really be done to cure the fish of the parasite, though dipping it in fresh water can help to reduce the number of parasites that burrow in (fish handle this fresh water bath better than the parasites do, so newly attached parasites may fall off before the fish must be returned to the saltwater it needs to survive).
Once the parasite is done growing, it releases from the fish and drops off the fish to settle into the bottom of the tank (which might take up to 8hrs). There, it begins a process of reproducing which can last up to 72 days. Following that period, it releases the "babies" into the water, which is back where we started. The babies (theronts) have about 48hrs to find a host before they run out of energy and die.
When you look at all those times, you can get a picture of where our treatment timelines come from. The "76 days fallow" starts with the assumption that you have mature parasites (protomonts) floating about in your water just fallen off the last fish you just removed from the tank. If we assume they take 8hrs to settle (round up to a day for easy math), then 72 days to reproduce (as a "tomite"), then 2 days to float around looking for a fish before they die - 1+72+2 = 75. We generally suggest adding a day or two so that there's a margin for error, so the common fallow period suggested is "76 days". You could do less, but you risk not eliminating the parasite from the tank - which makes all the effort spent meaningless.
Now as to why there aren't better treatments available;
One problem is in the hardiness of the parasite relative to the hardiness of the things we care about in our tanks. Corals don't handle copper anywhere near as well as fish. Nor do inverts (snails, crabs, clams, etc.). Even some of the beneficial bacteria we cultivate have issues with it. That makes it tough to treat in a tank where all that exists. There are other medications and methods out there that can target the parasites (Chloroquine Phosphate, for example), but again - those would harm the more fragile tank inhabitants as well. There are just no known weaknesses that the parasites we want to eliminate have that don't also exist in the things we're trying to save.
Another problem that's more recent (and complex) is that these parasites - for many reasons beyond my ability to list here - exist in the supply chain. It's not that we're pulling fish out of the oceans with these parasites, it's that the fish are added to tanks where the parasite exists and are infected. For you or I to hold a tank fallow for 76 days, it's no small effort. For a distributor who might have hundreds of tanks all within close proximity to each other (the parasites are suspected of being able to travel up to 10ft via aerosol transmission), getting the tanks clean would be nigh-impossible without a risk of going out of business. And even if they did get their tanks clean, any new fish may be infected and so the problem continues. That's why many distributors (and then retailers) are running low-levels of copper in their holding tanks. The lower levels allow fish to be added/removed without the acclimation period, but helps to keep the parasite populations down. It may also be creating parasites resistant to copper, however, so the problem is likely to get worse before it gets better...