Is the fast of one who felt something in his mouth without certainty, and swallowed his saliva without feeling anything reach his throat, valid?

Your fast is valid, and there is nothing against you, due to the lack of certainty that there was anything in your mouth to begin with, and the lack of certainty that you swallowed it, had it been there. The default is that the fast is valid, and it is not invalidated merely by doubt.