I found your introduction to your book very informative and intriguing based on the questions you are raising. In the beginning I was kind of lost as to what you were trying to research when it came to qualitative methods. Towards the end of the introduction your main points were listed and I thought it was interesting how you laid out each chapter, to imply that each question would eventually be answered by a different researcher. When it came to being "trustworthy" or "authentic" and the privacy boundaries that are broad I think that there are easy answers to this problem.
Later explained in your intro you state that, "method is not a recipe for success, but a means for an argument", quoted from Sally Jackson. I think that to wonder about the boundaries of trustworthiness and authenticity isn't that much of a problem for the internet world. Researchers as you said, do not have defined methods, which means methods can always be altered. To figure out the realness of research is to be able to prove it. If research is done face-to-face it still has to be proven, so why would the internet be any different.
Also when it comes to privacy I think each person can define their own boundaries, it is not so much the researcher that has to do that. Each person posts a different level of personal information, if it is public than it is public. People have the option to keep their lives private, and if they want to do so they can. Many sites have privacy settings, which allow others to see only what one chooses they see.
It is up to the researcher to define their boundaries, it has always been a researchers job to do so. I agree that the internet is intertwining many different medias together, making it more of a challenge, but I don't think it is impossible. The fact that researchers can define their own methods is what makes it possible to still conduct research even if the medias are merged.