Yes, Keotum's will detoxify poison, effectively it works the same as Neutralize Poison, at least to my reading. Slow Poison goes further, it actually retroactively cancels death from poisoning (in fact it will even do so several turns after death). AFAIK nothing else in AD&D does that, and the effects of MOST (there are a few exceptions) poisons are INSTANT, you fail the save, you are dead, do not pass go, do not collect 200gp. At that point it is TOO LATE for ANY other nostrum except Slow Poison (normally then followed by some true permanent curative). Honestly I think it is one of those readings that isn't particularly intended, but the haphazard writing of AD&D leads to a lot of these weird cases. I think Neutralize Poison is MEANT to be a more potent spell, and frankly I always ran it that way, but you cannot count on that, there are plenty of DMs who are out to rule you dead, not to cut you any slack at all.
Yeah, scrolls are really kind of ridiculously expensive, but they are also super useful, so...
Well, Sweetwater might also work, but that will depend on your DM being a bit friendly IIRC. Mainly it would have been nice if poison had been slightly more realistic and took at least a couple minutes to finish you off. Even a massive dose of sodium cyanide doesn't kill you in a single instant. Basically every poison monster attack does. 4e's approach of assessing poison damage each round is a bit more interesting, though few of the poisons in that edition are very dangerous.