The same reason when you vomit you don't end up with a dissolved throat. Human stomach acid is an very potent but is mixed with bases in order to keep from becoming too acidic. In addition to this there is also mucus that is produced by your stomach lining. These are all vomited up along with the acid. The acid would also be further neutralized by the mixture of gems that have already been digested.
The throat is partially dissolved when the act of vomiting has occured, thus the tingling sensation felt within (or that could just be things going up, not down). I'm not quite sure the other stuff matters so much, unless there is a layer of mucus already on Twilight's back protecting her. Or
Why are the comments filled with Spike haters? Why hate on Spike? Seriously. I find the bond between Spike and Twilight to be entirely unexplored except save "Owls Well That Ends Well". If you don't have anything nice to say then don't say anything at all. It's as simple as that.
Because it makes their relationship one of Mother/Son, and she's far, far too young at the time she gets him to be a mother. Being a mother entails not only certain responsibilities but a particular relationship dynamic which will persist throughout their lives. When you sign on to be someone's father/mother, you are that entity forever, and your failures in all arenas are the standard by which one is judged.
For a child to take on the responsibility of being a "mother" to a baby? Yes, that is too demanding and honestly a little disturbing, because it implies that NONE of the other adults in Twilight's life stepped up to be Spike's mother and left the responsibility to him. Despite what Equestria Girls would have us believe, Spike is not a pet, and raising a child is not the responsibility you give to another child. There's a reason "babies raising babies" is a terror-filled phrase.
That Twilight is a provider I have no doubt. That Twilight is closer than a friend I also do not doubt.
However, their relationship on every level does resemble that of an Older Sister/Younger Brother. Perhaps Celestia is more properly Spike's mother.