As I have grown up I have seen how as Science has moved away from explaining how the Universe works (e.g. Gravity) to how it began, theories have become shakier and shakier and "educated guesses" have become more and more prolific. For example, scientists do not know for certain what caused the Big Bang, despite many guesses and possible explanations, yet it is promoted as the most widely accepted theory.
Even in your example scientists are doing the same thing they have always been doing. Creating workable models of reality based on the observable evidence. The reason why the Big Bang is the most widely accepted is because it fits best with all the available evidence. If a new model comes along that can demonstrate that it fits the available data better, you would see scientists abandon the Big bang model for the new one. It doesn't matter if they are asking how the Earth began or how the universe began. "Educated guesses" are only a first step in the whole process. If we have more of them that's actually a good thing. It means we have found more things we don't know and still need to explore. Which is exactly what science is for.
As for certain I'm not sure what you mean. In science certainty works in degrees, not absolutes. So are the majority of scientists certain that the Big Bang model is currently the most accurate? For the majority, yes they are.
Will we ever know for certain? I do not believe so. We can wait untill we die, and find out then for sure.
What we do is create the most accurate model we can based on the available evidence and continue to revise that model as new evidence is made available. You have to keep in mind that all scientific knowledge is tentative an provisional to allow new information to be added.
I should hope not, but it is inevitable that they must make hypothesis' to explain things for the time being untill they know things for certain, and it is only natural if their preconceived ideas creep in.
While a hypothesis can be just an educated guess, it would be more accurate to call it a prediction. A prediction based on what we have observed but has yet been verified. The nice thing about science is the process is self correcting. This help to eliminate those preconceived ideas that creep in. For instance the requirement that new information be verified by an independent third party, the use of blind testing or how the conclusions must be backed by objectively verifiable evidence (opinion is not fact).
(BTW I am back from the grave... am I anonymous again yet?)
