As for point 1, see Vnovak's excellent answer, not least the superb differentiation between critique and consulting.
On point 2, the thing to keep in mind is that "like" and "dislike" are not criteria you should use to judge any design. "Like" is subjective, based on personal idiosyncrasies, taste and personal experience. Worse, saying what you like or don't like isn't useful. Rather, consider the business problem that the design is intended to solve. Bill Muggeridge, director of the Smithsonian’s Cooper-Hewitt National Design Museum, said this in a Washington Post article (via Design Talk):
“One of the big differences between art and design is that art is mostly about commentary — it’s making a statement that you’re expecting other people to contemplate and be moved by, emotionally, or altered by, in terms of their perceptions.
“Whereas design is really about solving a problem that makes something more pragmatic, and useful, and valuable or valued, and of course you can add qualities of aesthetics to that, that make it also a delight. At the same time, if it fails on the functionality side, all is lost, whereas if it fails on the delight side, it might still fit into a lot of people’s lives in a satisfactory if not an exciting way.”
Does the design accomplish that, or not? Can you say why, in a short paragraph or two? If you can't, step back for a moment and reconsider. As a designer, you should be able to articulate why an element is or isn't effective, does or does not contribute to the purpose of the design as a whole. The medium (print, web, mobile app, video) doesn't matter here: the same question applies no matter whether you're talking about a billboard or a master shot for a movie scene. John McWade has another excellent post that makes this point very well.
Not only will you find this an incredibly useful exercise, one that will help you to look at your own designs less subjectively, it frees you from the ethical dilemma. You will know for yourself whether the design is effective, independent of your personal taste, and you will have learn a new skill in the process.
You will also be able to relax on the subject of whether you should take on the job of redoing the client's identity materials.
On #3, if you really don't want to get into it, a simple "I don't feel comfortable commenting on the work of another designer" should be understandable.