Not all of them work. It's a bit naive to say they work, because most of them don't. Not only to the spells have to work, so does the wish. Magick is what nature can do, if you wish to have antlers it won't work. If you wish to make a new friend, it could work with the right spell. Though there are a million wish spells, and I'd say about 1 out of 10 work. Just use common sense to sort through the fake spells.
Depends on the spell. If it is something that sounds reasonable then it probably will but if you are trying a spell to give you wings or a tail then it probably wont. Just keep the spell realistic and it should work just fine.