Here is how I see it. For a word to be an adjective, it must be describing some object. If you say, “The pot was watched”, watched is not a property of the pot. There is an irony in “A watched pot never boils”. The sentence goes against our perception that being watched has no effect on the pot and should have nothing to do with whether or not it boils. In the last example, I can see “tarnished” used as either an adjective or passive voice, although the more common use would be as an adjective. The focus would ordinarily be on the current state of the pot rather than acts of tarnishing that had been made on it.