First you are correct in assuming that you need to beef up the 3/4" subfloor, so that the hardwood does not flex and squeak eventually. But I have found that only by adding 1/2" plywood does this work in the long run. 3/8" might work, but the effect might be marginal, and 1/4" won't add any stiffness at all. So add the 1/2" plywood and be sure to use squiggles of the new urethane construction adhesive on and between the joists as you nail this new underlay down to the subfloor. Nail or screw so that the these fasteners go into the joist, and run a second row of shorter fasteners ( screws for sure ) between the joist. This second row of screws should not penetrate the subfloor, as this is your only real moisture barrier. But it will help the 2 plywoods act as one solid sheet.
There is no point to laying a vapor barrier just before you nail hardwood down, you will just be punching a thousand holes in it as you install the hardwood. Just rely on the waterproof nature of the plywood, and the fact that your 2" flooring nails will only penetrate the total subfloor thickness 7/8". True there will be a few face nailed rows that will penetrate both the plywood layers, but that will be limited.
Always use a exterior grade of plywood, and this will prevent any dampness from getting to the new hardwood from below. Allow even the plywood to acclimatize. Overlap the seams of the subfloor with the underlay.
Try to get the relative humidity lower than that 70% in the space below the floor ( I hope I understood this is what you said ). It is the difference of moisture content between the top and bottom of the hardwood floor that can cause it t cup or crown. Ventilate, dehumidify, heat or air condition any crawl spaces under hardwood floors, to minimize this.
Oh, and NEVER, NEVER, NEVER lay a hardwood floor with screws as you are about to do. Read my article about the Primatech Floor nailer, in the Product Reviews section of this site. Don't worry, it's free. When a hardwood floor is TOO well fastened ( in the article I mention the floor staples doing just this ) it will no longer be able to move seasonally. What may result is tilted boards and even cracked tongues in some cases. Stick to the good old nail technology, read the article, and you see what I mean.
Now as to your last question, I hope you have read my article about Prefinished Floors : Which to Choose, also a free read in the How To's section of this site. There are so many brands out there, that there is no way to keep up with the varying quality of milling and finishes in the Prefinished sector. A lot of the exotic woods are now milled and finished in the factories in the Far East and South America, so you should really scrutinize this material BEFORE you buy it.
I have found that in general however that the big box stores ( no names ) will stock only the most mediocre material. They will have the best prices, but there is always a reason for this. And the species of wood you mention, is not it's real name. Find out the genus and species for this wood, and I can give you it's pro and cons. If the store does not know the scientific name for this wood, or will not tell you, I would pass. You may find that later when you need some repair material, it is no longer being imported. Sometimes a whole room may need repairing, you've been though this already.
And the hardness of this wood may come back to bite you later. When the finish on this wood wears out ( and it will, I am now sanding prefinished floors installed only 4-6 years ago ) the floor mechanic will have to charge double the going rate to tackle your floor. It will prove to be a difficult wood to sand smooth if it's an oily tropical wood. Also some of the really resinous woods will not take a conventional finish well. Drying and peeling problems can occur depending on species. So get back to me on the species, and I will give you a more definitive answer.