Typically grammar is governed by style guides, in the case of what Elefante pointed out the final say could be that of any reputable dictionary, for example, Oxford or Merriam-Webster's. Having standardized rules is useful for formal occasions and communication with and between newer speakers of a language. From a purely Kantian or utilitarian perspective not complying with grammar rules is not unethical. Not complying can make what you write harder to understand and sometimes make you less likely to be taken seriously, though these are situational.
Yes. Grammar changes through the contri