Well it seems that for you, "X" is definitively true if you believe X is undeniable. Fair enough, but what you are describing is merely a measure of perceived certainty. There are probably people who really do believe that there is a Jewish conspiracy to take over the world and they may well be as certain about this as you are about your 'definitive beliefs'. I do not deny that (1) people can feel certain about things; (2) the hypothesis about which they are certain may be "objectively" true. But if we are talking about an "inner feeling of certainty", I think it's reasonable to be skeptical about such claims, but not to the point of insisting that what the person is certain about cannot be true.
Hmmm. I politely suggest that this choice of example, and the way you frame it, suggest that you indeed may have a distorted view on the nature of scientific inquiry and the robustness of the scientific method. For example, it is clearly profoundly misleading, at best, to suggest that scientist "assume" dinosaurs lived millions of year ago.