I can't answer the part about tooth enamel, my teeth were ruined by gluten to begin with, and in the over 4 yrs that I have been taking B12, I do not think they are any worse.
As for swallowing, or letting it dissolve, there is a lot of controversy over it. Some believe it doesn't matter, some believe it is very important to let it dissolve. I believe letting it dissolve is best, some does get absorbed, if you just swallowed, it's all in your stomach at once. My neuro said it comes in a nose spray now, but I haven't checked into that. Some doctors believe it's best to inject it, which bypasses the stomach altogether. I guess it's whatever you feel is best. I can say, in 4 1/2 years, my B12 has slowed my neuropathy progress, and in some instances, it has improved it, so I will continue to take it my way.