All your examples are grammatically correct, but they don't mean quite the same thing."X is it" is usually used to mean "X is the selected one" or "X is the only available choice" in a fairly emphatic way.
Like, "Who is stuck with cleaning the toilet today?" "You are it." You could also say "It is you", but that would be just identifying the person, while "You are it" carries the connotation that you are the unfortunate loser of the selection process.
Or, "Who is going to lunch?" "We are it." This emphasizes that "we" is a more limited group than was expected. Perhaps others who were going to join us are too busy.
"It is I", etc is imply identification with no connotation of exclusiveness.
Note that in spoken English, people almost always say "It is me" rather than "It is I" and "It is us" rather than "It is we". This is grammatically wrong but very common. We've had some arguments on this site before where some of the residents insist that the fact that it is so commonly used makes it right by default. I don't think we need to rehash that argument. Just accept that by the traditional rules, you should say "It is I", but that most people in fact say "It is me", and go where you like from there.
RE the last question: It's not that you use the same form of "to be" both ways with "he", it's that "it" and "he" take the same form of "to be". That is, what verb you use depends on the subject of the verb. "I am", "We are", "You are", "He is", "She is", "It is", "They are". The subject of the verb is irrelevant to picking the correct form. So "I am it" versus "It is I" because "I" takes "am" while "it" takes "is". "He is it" and "It is he" because "it" and "he" both take "is". If you find this confusing, imagine the verb was something other than "to be". If you said, "He eats a cake", you would use the singular verb because the subject, "he" is singular. If you said "They eat a cake", you use the plural verb because "they" is plural. The fact that "cake" is singular has nothing to do with it. "He eats cookies" -- you still use the singular verb even though it's plural cookies.