Nope, hot water is what works best for soap nuts. Warm water also works reasonably well to release the saponin but I'd stick with hot for diapers. It's true that the hotter the water the more quickly your soap nuts will get 'used up' but that's how they clean. Cold water doesn't work at all for soap nuts, if you're using them for regular clothing and wash in cold you have to soak them in hot water for a couple minutes first to release the saponin (then throw both soaking water & nuts in the wash).
I'd say 3-4 should be plenty, it's been a while since I used mine but if I remember correctly I used 3-4 halves per bag. I would use the bag twice for anything washed in hot water (ie diapers) and I'd use it maybe 3-4 times if I using cold water and just soaking the nuts in hot first.