Qs 1 & 2: Yes, no, maybe.
Q 3: No, for the vast majority.
My rules of critique:
Be as critical as you want, but be constructive and positive about how you present your critique when asked.
Talk about the photos, not to the photographer. Don't presume that you understand the photographer's intent — frame your critique in the context of what you see and how it affects you.
Take enough time to frame your thoughts about a photo rather than blurting out a critique too quickly.
Separate the notions of a photo's content from commentary and critique on its technical merits. Although the two things do have to support one another, sometimes a very fine photo is simply in need of better rendering, and sometimes a beautifully rendered photo simply lacks enough story or intent to be considered good.
Say "thank you" when you get a nice comment, if the situation is apropos.
Say "nice job" if you like what someone did and feel you must add a comment, but haven't much specific in mind.
Don't say anything if all you are going to do is say "that sux!"
NEVER modify and re-distribute someone else's photo without asking them off-line first if it is all right, or unless the person who posted it specifically granted viewers to modify and play with it.
G