If a fish exhibits dropsy it does have an extremely high mortality rate, yes.
Are there any particular lumps or bumps anywhere on this fish (not counting raised scales)? Unfortunately usually when fish start exhibiting this many different parasites and dropsy it often also means that they are having organ failure, often multiple organ failure. Liver tumors unfortunately do occur fairly frequently in bettas, and I'm afraid this is totally incurable, and absolutely fatal.
The unfortunate problems with both Melafix and Pimafix are that they are simply extracts from various trees native to a betta's wild habitat which help neutralize many of the bacteria there. However, these meds are best as preventative measures I've found, and in my personal experience, once the fish is ill, these are pretty much useless. Maybe someone else who has had better luck with them can help.
So let us go down the list, although I will go ahead and say right now, it is not looking good for your fishy that he is displaying this many issues. His odds of making it through this are not great.
Velvet - best recommendations are to keep up the salt dosing, keep the water warm (you could even bump it up to 84 to help kill this stuff faster) and keep the tank on the darker side. If there is a tank light, turn it off. The parasite that causes velvet is kind of funky in that it also has chlorophyll (stuff in trees and plants) and it needs some form of light in addition to the nutrients it is sucking out of your betta to survive. Keeping the tank in low light helps kill off the velvet. And keep up on the water changes! Clear clean water is essential to finally remove all of the velvet parasites from the tank.
Ich - so contagious. I rarely have had to deal with this because keeping aquarium salt in a tank is a great preventative of this. Alas, that doesn't work so hot when you bring a fish home who already has ich. Raise the temp to 85 (the little buggers don't like that temp and let go of the betta then) and then add 1 drop of Aquarisol per gallon, per day until it's all gone. Aquarisol is a great anti-parasitic and kills off the ich. Oh, and clean water!
Bloating/Constipation - fast you fish for a day and then try feeding them daphnia, or a single, boiled, de-shelled pea. Beware! It may not just be constipation. Because of the amount of conditions this betta is suffering from, it is highly likely that it may have developed a swim bladder disorder, and is now unable to regulate it's balance. Is he laying on the bottom of the tank a lot? This may have even progressed to a point in which the swim bladder has shut down, in which case it is now becoming necrotic, and by that point your betta is already beyond saving and will die within a day or two.
Finrot - Many people often say that fin rot is the result of dirty water, and I'm inclined to believe them. Likewise, the most often recomended and simplest cure is simply frequent water changes and clean water. However, in severe cases, or when the fish is already suffering from something else, it is also generally recomended that you treat it more aggressively. Ampicillin or tetracycline are the most effective known medicines for dealing with fin rot (and its more severe cousin body rot - what happens when the fin rot eats the fins all the way back to the body and begins eating the body). I recommend Ampicillin generally because it's more easily available in my area, and because in your particular listing, Ampicillin is also handy for some of your betta's other issues.
Fungal infection - I've found that Fungus Eliminator by a brand called Jungle is pretty good stuff and will knock most of it out in a week. Be careful though, in he crystal form, when you put it in the water. Some betta apparently think this is a food and try to eat the crystals 0__x This is not good and will kill them (learned that one from experience).
Popeye and dropsy - as I mentioned earlier, neither are a disease themselves, so much as a symptom of something else being wrong. Popeye is likely a result of the Velvet and fungal infection. Again, it is only a symptom, not a disease itself, and it's best treatment is simply clean water and a dose of Ampicillin. Although I have seen cases of popeye which have recovered simply from religious tank cleaning. Dropsy is generally a sign of organ failure, generally the kidneys or liver of a betta, and this is also why it is considered to be so fatal. Just like in humans, you loose kidneys or a liver, and you're gone (and sorry, there's no betta organ transplant surgery).
Those things swimming around in the water and attacked to your fish? Those are external parasites. Enough salt in the tank and they will generally release their grip on the betta and go die. Thorough water cleaning is required though to make sure you remove all of the parasites and their eggs. The unfortunate sign here is that if your betta is exhibiting external parasites, he most likely also has internal parasites (well I'll be frank and say I'm rather sure he has them considering the dropsy) and these are much more difficult to treat.
Scales and body parts falling off? This is bad, to say the least. While I've never observed this in a fish personally, my best correlation I can make is to humans and necrotic tissue decay, in which the body is simply losing its ability to properly maintain its entire body, and as such is sacrificing 'less vital' areas in an attempt to maintain proper circulation to other parts of the body. In your fish, this may well be causing the loss of scales and ... tail bits? Honestly I've no real cure for this in a fish, if this is even how.why it is happening.