Well, "doubt over the victor" in Zoroastrian mythology isn't in doubt, but for some (like the Norse), the "bad guys" win Ragnarok, and for some (like the Greeks vs the Titans), there's some doubt if that "final battle" will ever come. That may not specifically be "doubt over the victor" exactly, but it's an indication that the good guys don't universally win.
Yeah ... that's a different critter like I said.
Maybe it helps to add some terminology for clarification: the religions where a "good guy" fights against an equivalently powered "bad guy" could be called "oppositional dualism." Religions like Taoism (or even like my own Wicca) where there are two opposite but complementary forces could be called "Complementary dualism," because both sides of the coin are needed for either to work. And religions like Christianity (Judaism and Islam to a lesser extent) could be called "Unequal Dualism," because the bad guy is definitely much weaker, and his fate is pretty much sealed.
A good fictional example of "Oppositional Dualism" would be the Valar in the Simarilion: the "good guys" (the Valar) fought against the "bad guy" (Morgoth), but Morgoth was actually himself a Valar--indeed, he was stronger than any of the others, even Manwe. Since Illuvatar wasn't going to get directly involved in the Battle, it was up to the Valar ... and there was considerable doubt for quite some time over who would win.
No, because even at that, it's not the power of Satan that necessitates "salvation," but the consequences of our own sins. Satan's just along for the ride as a tempter--yeah, he's powerful, but according to the literature even a single Christian can make him flee.
No, I think the closest it gets to what you're talking about is Zoroastrianism, or the Christian heresy of Manicheism. When Satan was adopted by Judaism (and thence by Christianity) from Zoroastrianism, he really got weakened, and is no longer even remotely equivalent to God.