If someone inadvertently intended to eat on Yom Kippur, and realized his mistake after reciting the beracha on food, he should not even taste a little bit. It is better to have said a Beracha L’Vatala (in vein) than taste even a minute amount of food. He would then say "Baruch Shem Kavod Malhuto L’Olam Va’ed" to rectify his transgression. This is a function of the grave severity of eating on Yom Kippur. On other fast days, it would be preferable to taste the food, rather than say a Beracha L’Vatala.
Swallowing saliva is not considered drinking, and there is no issue with doing so. Nevertheless, there were great Rabbis who accepted upon themselves as an extra stringency not to swallow. Rav Natan Wachtfogel of Lakewood Yeshiva would sit in the front of the Bet Midrash and secrete his saliva into tissues until his mouth dried up.
Chewing gum, even if flavorless, is prohibited on Yom Kippur.
Not only is it permitted to smell fragrances on Yom Kippur, it is considered a Mitzva, even though it is enjoyable. Doing so allows a person to complete the requisite one hundred Berachot that should be recited every day. The normal routine of Yom Kippur is missing Berachot because of the lack of meals. The Beracha on fragrances can be made several times a day. It is also permitted to inhale snuff. If one inhales perfume or cologne, Hacham Ovadia’s conclusion is that a Beracha of Boreh Mineh Besamim is recited. It is permitted to recite the Beracha on fragrant fruit, "Hanoten Re’ah Tob BaPerot," and the Hachamim were not concerned that one might inadvertently eat it.
It is preferable to make a Beracha L’vatala than to taste a minute amount of food on Yom Kippur.
It is prohibited to chew gum on Yom Kippur.
There is no issue of swallowing saliva.
It is considered a Mitzva to recite a Beracha and smell pleasant fragrances, including snuff, fruit and synthetic perfume.