The Silmarillion contains much backstory on Middle-Earth and the beings that inhabit it and that is where most of the information of the Valar and Sauron and other Maia come from. For example, Tolkienites correct me if I'm misremembering, Sauron literally bred the first dragons. Also, the Balrog that Gandalf will not let pass in LotR is an example of one of the lesser Maia that followed Morgoth and then Sauron, a deity, or demon if prefer, from before there was such a thing as time or planets.
You are assuming that Voldy's spells would have an effect on Sauron. Please correct me if I'm wrong fellow Tolkien fans, but I don't remember any instances of Sauron being vulnerable to magic that wasn't coming from the Valar or Maia.
You misunderstand, the Ring isn't a "horcrux" (I hate that word, the correct term, for those who's fantasy knowledge is based on Potter, is 'phylactery' which descends from the Judaism meaning of a small leather box containing parchment that has been inscribed with protective scripture, to the Christian meaning of a container holding a relic, to a general meaning an amulet, charm, or other safeguard against danger, to the D&D codified fantasy definition of an object storing part or all of a persons soul energy so as to safeguard them from death ... Anyway...). Since Sauron isn't mortal, he doesn't have a soul so the Ring is not a phylactery, the Ring is literally a container for a large amount of his power and, following well established fantasy/occult rules, by storing some of his power in an external container he is more powerful when his internal power is combined with said external power.
I think we need to clarify, as I've pointed out repeatedly, Sauron is a deity and as such he has various spheres of influence. A sphere of influence is a concept or an aspect of reality that a deity has total or near total control over. For example, one of Zeus's spheres is lightening, which in ancient terms means he has complete control over lightening, he can make it strike specific places, he can make it not strike at all, and he can even manipulate it as we would playdoh and use it as one would a physical object. One of Sauron's spheres is deception and the Ring, being a container for enough of his power to create a form of sentience, gains benefit of that sphere. What that means in the context of this debate is that the Ring can tempt any who seek power into coveting the Ring and its power. The effect is so strong that Smegal, the proto-hobbit that would become Golum, merely looked upon the Ring and was utterly corrupted to the point that he immediately murdered his best friend to take possession of it. Only someone as devoid of ambition as a hobbit and as pure hearted as Bilbo or Frodo is able to own the ring without being instantly corrupted and even they were corrupted over time. Being good or evil doesn't really matter as the two most Good people in Middle-Earth, Gandalf and Galadriel, dared not touch, much less possess, the ring for fear of their good intentions paving a road to hell. There is absolutely no way that Voldy, a power hungry dark lord, would be able to gain the ring and not be corrupted to Sauron's will and he most certainly would never be able to destroy it. The Ring is not a mere magic item, it is a deity created Artifact of Power.
A patronus may be the only thing that a mortal wizard can use to fight off a dementor, but the Cosmic Magic of fallen deities, like the balrog, and the Ancient Magics of necromancers like the Ringwraiths is not mortal magic and would be more than enough to battle and defeat them. Harry's patronus is basically equivalent to a modified Bigby's Forceful Hand (I'm sure I'm forgetting a more appropriate spell, but whatever, this works for the example) against dementors, which is a 6th level spell, I'll be generous and just assume that Voldy has decent access to level 9 spells. While Ringwraiths, due to their ancient nature and the standard fantasy/occult/trope rules state that magic was always more powerful in the past, would have access to level 10 spells. In short, each Ringwraith IS a wizard that is at least if not more powerful than Voldy himself.
(For those unfamiliar with the AD&D spell level progression, each spell level is basically exponentially more powerful than the previous, kind of like richter scale for earthquakes and level 10 represents the highest level of magic that one can reach without being a deity, so much so that with the proper spells and rituals a wizard with level 10 spell ability can trade places with a deity and assume their power, though the one time it happen in a different universe it almost destroyed that universe and caused level 10 spells to be taken away from wizards there forever. ... Anyway ...)
As for killing a Ringwraith, it doesn't take a women, in the story it was two people, happened to be a man and a women, but that isn't a requirement. The Ringwraith your referring to was killed by swords created by the Westernesse, descendants of Middle-Earth's version of Atlantis which was populated by humans who descended from an ancient elf back when elves were more than the shadows they have become. In other words, the swords that killed the Ringwraith were powerful ancient artifacts from a time when humans were more akin to low level demigods than the normal mortals of today. Aragorn is the best known of those descendants of descendants of descendants and he was a pale version in comparison. I'm not an expert on potter-verse, but I believe a good comparison between the ancient and modern swords would be the first wand versus a normal off the shelf variety.
To your mention of Isildur's luck and the potter-verse luck potions. Isildur wasn't lucky in the sense of being lucky at cards, he was lucky in the sense that by virtue of his lineage he was the only one that had even the slightest chance of defeating Sauron and that was with his special sword and even then he only succeeded because Sauron got overconfident and after all that Isildur still failed because he was corrupted by the Ring.
To put this whole thing into a different context: The question is basically the equivalent of who would win in a war between General Patton (or Genghis Khan or Sun Tzu or whoever you think it the best General ever) with the best army the earth can field and the Archangel Michel with the host of heaven behind him. Magic or not, horcruxes or not, Voldy and his crew are just a bunch of mortals. Sauron and many of his crew are literal deities. Mortal magic is a joke when we are talking about the creatures that literally sung physical reality into existence.
EDIT: That is way longer than I intended, which means I'm starting to babble and it is time for bed.