Well you're not entirely incorrect with that assumption. What we call science today is actually the Scientific Method Which is a much more skeptical approach to science than the earlier methods, hence the credibility. I like many others agree that the fees built into the system is quiet absurd and the process is not perfect, but currently that is the only legit way to get others evaluate your research.

Fair point, I should specify "modern science". There's quite a gap of scientific quality between traditional medicine and modern science based medicine for example.

No, peer reviewing can happen in many ways. But it needs to be public.

Sending letters also allows for peer reviewing.

Was zero science conducted before the ~1600s then?

I mean, yes. The framework of studying things that we understand as science did not always exist.

Possibly. I can't come up with any major results that wasn't either logic, engineering or tradition. But it's an interesting question. What might count as science before then?

not as a discipline. If you publish an experiment to the extent it can be reproduced, it is science, so its happened before but in a less intentional fashion

Before the 20th century most famous physicists referred to themselves as “natural philosophers,” not scientists. The P in PhD is for philosophy. The word “science” refers to a modern social phenomenon, a sort of peer-review methodology that generates shared public knowledge.