My mystery TRIES to float, but she's too big now. She does do this kind of side float thing when she is on the bottom, or I take her down to feed her, but she will press those bubbles out of her shell eventually. One way I can usually get her to come out of her shell is to hold her in my hand under water, or hold her up against the glass.
Also, if you want a good cheap shell supplement, get some cuttle bone (bird stuff) and use a cheese grater to make it powdery (works better if you get it wet first), and put about 1/2 tsp in every 2 weeks or so. You can also just bury half the cuttle bone in the substrate. I can't believe how much better my snails shells look just because of the extra calcium.
Like stated before, they will smell terrible when they are dead, and you will know.
My malaysian trumpet snails float on the surface ALL THE TIME, and so do my ramshorn quite often. The only ones that never float are the assassin snails.