You’re not the only one. Philosophers have been asking that question for thousands of years. Of course, there’s the Kalam Cosmological Argument which takes us back to the ultimate First Cause. You may also find this blog post by Christian theoretical physicist Aron Wall interesting – he says that when atheist scientists such as Lawrence Krauss talk about the universe coming from nothing, what they are describing as “nothing” isn’t quite as “nothing” as they think it is.
Well. @Bill_II said it already: dating methods are cross-checked against each other – so no, there aren’t really any presuppositions involved here. It’s testable predictions again: if this uranium-lead date is reliable, then we should get the same results from rubidium-strontium, lutetium-hafnium, potassium-argon, or even continental drift.
Well yes, any honest scientist will admit that there are plenty of things that they don’t know. (Take a read of the book “We Have No Idea” by Jorge Cham and Daniel Whiteson if you’re particularly interested.) But the fact that there are things that we don’t know doesn’t mean that the things that we do know are wrong. Especially when they are confirmed by large quantities of rigorously cross-checked measurements.