I think that for game purposes, Bluff, Diplomacy and Initimidate would be better balanced by re-defining them as:
Bluff: Getting the subject to do what you want by making him trust you.
Diplomacy: Getting the subject to do what you want by making him like you.
Intimidate: Getting the subject to do what you want by playing on his fears.
Under these new definitions, convincing someone of the truth when you have no proof is not a Diplomacy check, it's a Bluff check. If you're telling the truth, then the DC is simply based on how improbable your (true) story seems. If you happen to be lying, then you
also have to beat the subject's Insight check.
Similarly, painting a dire scenario of what could happen if the subject does not do what you ask him to do should be an Intimidate check, not a Diplomacy check. Threatening to personally do violence to the subject or his family members is only one way in which Intimidate could be used, and the one most likely to have only a short-term effect and/or to backfire. Suggesting to the Duke that the gnolls are going to invade his domain next unless he sends troops to help the neighboring barony fight off their invasion would be an Intimidate check that would not have short-term effects or make him ill-disposed to the intimidator.
However, if these new definitions are used, the skills may need re-naming to better reflect what they actually do.