رضا:
1. There is no grammatical problem with using the word loosely in that sentence, but it is a somewhat odd thing to want to say. I agree with ScarletRose that forms (1) and (3) sound most natural. I also agree that a scholar probably wouldn't use that sentence -- not because it's grammatically wrong, but simply because a scholar who wishes to insult someone can probably do so with more sophistication than the average twelve-year-old.

2. The reason why you wouldn't say something like "He may probably have gone out" is not that may and probably mean the same thing, but that they mean different things. If I say "He probably has gone out," I mean that I believe he has gone out (although it is possible that he has not), or at least that I think it is more likely that he has gone out than that he has not gone out. If I say "He may have gone out" I simply mean that it is possible that he has gone out; I'm not necessarily saying anything about how likely it is. If you used both words together, I wouldn't know what you were trying to say.

3. Regarding the word already, (1), (3) and (4) are all fine. I wouldn't say that (2) is strictly wrong, but it sounds strange. I think this is because in (2) it feels like the adverb already is attaching itself to may rather than to have gone.

4. Usually at the end of a movie you'll see the names of the director, producers, actors, camera operators, costume designers, set decorators, etc. This portion of the movie is called the "credits", because all these people are given credit for their roles in making the movie. Credits at the end of a movie are called "end credits"; credits at the beginning are "opening credits".