1) Contrary to the helpfile, it looks as though INT and WIS (no longer?) matter for squat with this skill
This may still affect how often you poison weapon, but I am a novice so I doubt it played a part in my failures. Failure rate does seem a bit high though. Of the 5 times I attempted, I failed three times.
2) Does it really eat away at weapons? Maybe only less resistant ones?
This does not seem to be the case, but I was only able to do it three times on "a blunt dagger" (whose worth is 1 copper). My dagger remained "usable" the entire time.
3) You get the echo "You feel less sick." sometimes. I think this happens when the poison on the weapon wears off, if (if and only if) you attempt to poison someone with it (and fail?)
I'm not sure that this is the case, but I finally got this message after a short while (10 seconds after poisoning my weapon). I didn't even fight anyone. If that's how long the poison lasts, I would suggest an enhancement to increase the duration. 10 seconds is really, really short to keep it on a weapon. Especially when considering the amount of water you'd need to carry to keep your weapon poisoned.
4) The poison does -2 STR rather than -4 CON? Is that all poisons, or just this one?
It doesn't appear to affect you at all. Of the three times I failed and "poisoned myself," I got no affects placed on me. I did, however, take damage. Of the three times I was able to strike my test subject, he said he saw nothing of poison and no affects were placed on him. I suspect it only adds to damage.
This land shall come to the God who knows the answer to War. -Ninety-Nine Nights