Etiquettes and content of review text for replying to reviewer comments?
The more you write the more people need to read before they get to the stuff they need to know, so I would keep the formalities short. I typically start my response letter with a general thanks to all reviewers. If one has been especially useful I will mention that there.

If there was a comment that happend in all (or many) of the reviews I will discuss that at the begining as well.

After that the letter becomes very "tabular", For each reviewer I just discuss point for point how I dealt with that comment. Usually it is just a sentence and the page (and line) of where that change happened in the text. Occationally it may make sense to write a bit more, and I do so whenever required.