I refuse to put snails in the breeding/fry tank until they are bigger and stronger.. I've seen both big and small snails walking on the bubble nests themselves and it terrifies me they will eat any eggs.. and they eat all my live plants.. so they are in a separate tank, and when the fry are moved to the grow out tanks I will add in a single snail - they create a TON of waste and ammonia.. the only "clean" they do is eat food from the fry on the floor, and dead fry.. .
I have clove oil.. just make sure to shake it really well before placing fish in there.. it has to mix up otherwise the fish will suffer. I have an asprin bottle (big one) with a tight lid that I use to mix it up in, before placing it into the deli cup with the fish.
Other than that, the only other way to insure no suffering is to either cut it's head off or smash it under a newspaper.. I can't do either :(
Most people will sell online - through aquabid, ebay, facebook groups, pet stores will pay pennies for them.. but that is a way to get rid of any that have not been sold after a while.. most breeders who breed a lot will cull very heavily to avoid a ton of leftover fish.
Using a drip system.. it depends on each breeder.. some start dripping the new water in when the fry are free swimming, some wait a few days. Depends on how clean the water is, live plants, etc I suppose. You don't want to be pouring in new water.. has to be painstakingly slow..
Form.. If you are wanting to show fish then I would look into joining IBC (International Betta Congress).. they will help you with proper form, etc. Also breeding techniques.