OK Frist off I said bettas come from rice patties. Yes secound not all betta's are from rice patties most all bubblenest spices come from stagmant water,with temperatures from 75oF/24oC to 95oF/35oC,water stained by peatanddead leaves,acdic PH in the rangeof4-5.5 low to moderate dissolved calcium hardness and low conductivity indicating few salts or calcium ions in the water. In these ditches,swamps,rivers backwaters,floodplains,rice paddies, and canals,the bottom is often mud mixed with silt,the water is less than 3 to 4 inches in deepth.
The rice paddies of Malaysia arn't nothing like the pictures of the ones on the page you posted most in Malaysia or shollower then normal. B. imbellis is normally a mistreated fish it is bred to fight then released back in to the wild. As with any betta I don't beleave they should be kept in a cup { Never said that never will } But even a gallon bowl is as deep as most rice paddies are. Not saying they should be kept that way either But most breders keep the femles in a box built for BEANY BABYS. A 4x4 square box. Again no don't beleave in it never will. I house all my females in a 55g tank. 3/4's of my males are housed in a 3gal bowl. Only 3 do I have in a 1g bowl this is because they don't get along with a singal thing.
As long as the bettas being treatd nicely feed ,water changed and kept in a disain't einvierment and not in a cup. I beleave there happy. Most any Betta only swims in a small area anyway.
Now ocean sounds to me like you've got a goodhandle on what's wrong with your. As for the snail getting what the beta has. I see noway they could,as they don't have a swim plater nor do they have a skils for dropsey. I've had great luck with primafix it's all natural and dosn't sem to hurt the betta.