I think the questions you're asking are good ones, and go to two separate issues.

One issue is the arbitrariness of many social conventions. We observe seven days a week because that is the convention, not because there is anything intrinsically "true" or "right" about it. (note the similarity between conventions and platforms - like facebook - that enjoy wide use as much because of network effects as because they provide a superior product compared to, say myspace or something else).

The other thing you get at is how we manage to put trust in anything, where that trust has nothing to do with arbitrary convention but because we expect there to be a relationship between a claim and reality. There the best I can say is that obviously we cannot all be experts in everything, and must place our trust in others for many things. Most of our thinking, reasoning, and experimentation HAS to be outsourced if we are not to go back to living very simple lives as hunter gatherers.

But still we can try to gain as good an understanding of modern technology and the science it is based on as possible, and try to bring our assumptions about novel ideas, and our faith in experts, in line with the things about which we do have some solid basis of knowledge. The idea is to build a basic competence in our understanding that's rooted in the demonstrable correspondence between our beliefs and reality, and then expand it outwards to encompass others' beliefs when they are congruent/coherent with our own.