Here are some answers to the best of My knowledge.
1. It depends on how much is used, and which substance. Toothpaste is more mild than Icy Hot or I'm supposing, Tiger Balm.
2. Toothpaste lasts 10-20 minutes, if a pea sized amount is used on the balls, and another pea sized amount is used on the cock. Icy Hot lasts a little longer, as it's really intended for muscle aches.
3. Never had anyone put any on their asshole. I don't think it lasts any longer or shorter on the cock or balls. I've never had anyone mention anything about their balls not being on fire when their cock was on fire.
4. If the sub likes to be hurt, there is some pleasure for him. If he's not a painslut, then there's likely little more pleasure than having Me mock him, and laugh at him.
I read your explanatory story (Ha, rhymes) and I tell boys to be careful not to get either Icy Hot or toothpaste on the head of their dick, as it's not intended for internal use and it's not good for things like that to be on mucous membranes. The fact the woman had Icy Hot inside her may have been what caused the seizure, or something else may have triggered it. I always tell boys to be careful when playing with things like this.
Well, I hope that helps you out some.