Is it a body of knowledge arrived at by means of the scientific method? https://en.wikipedia.org/wiki/Scientific_method
> 2. Who should decide it?
Each person for themselves. The whole point of empirical knowledge is that it does not matter who performs the experiment.
> Are there guardians of Science?
Yes, but they will tell you that the true authority is the real world: the results of experiments.
> 3. if every theory is wrong until proven right...
No scientific theory is right. Science does not and cannot prove anything. This is also something that the guardians of Science will insist on. Even the Standard Model (which has no known violations) has only failed to be disproved so far.
I don't understand the rest of Q #3.
> 4. Who and How is it decided that something is science or pseudo science?
There's a lot of hemming and hawing but it comes down to empirical experiment as the source of confidence in knowledge. (Also, math.)