"Redundant" involves repetition. In the following example, there are two examples of redundancy: "This blue, azure shirt is torn and ripped." Blue and azure are redundant, and torn and ripped are redundant. Note that these redundancy pairs do not include words that are exactly synonymous, but which are close enough in meaning that one would usually consider them redundant. Two points here: 1. Neither word in such a pair is necessarily the redundant one; either one can be considered redundant, depending on which one you consider to be the more important, useful, or accurate one in the given context. Commonly, the second word is considered the redundant one, but that is merely because the first word got a chance to establish itself before the second one came along; if you were revising the text, you might choose to keep the second, not the first. 2. The same word repeated ("this blue blue shirt") is an example of redundancy, but this is usually done for emphasis, or for poetic effect, and so is seldom saddled with the accusation of redundancy. Thus, "redundant" does tend to carry the implication of an unnecessary repetition.
A tip: To help you remember this, note that "redundant" begins with "re," as in "repetition." That piece of these words means "again."
"Superfluous," on the other hand, refers to something that is more than what is necessary. Think of water running over the rim of a glass when you continue to pour water into it beyond its capacity. The water over- (super) flows (fluous). Often something superfluous is so because it is needlessly repetitive, and this confuses the picture a bit. But in my opinion, "superfluous" is better used when the element is not repetitive, but is genuinely not needed, as in this example: "After George embedded the fence post in thirty pounds of concrete buried underground, the brick he balanced atop the post to hold it down was superfluous."
For real indeed has a kind of slangy/childish connotation, which I'm sure is why it was used in the title of that movie (a quick Google search reveals that the film is about a four-year-old who has a near-death experience).
However, it also does not mean the same thing as real: if something is real, it exists, while if something is for real, it is legitimate.
Merriam-Webster backs up this adjectival sense of for real (definition 3.2 and 3.3 at that link):
2: genuine "couldn't believe the threats were for real"
3: genuinely good or capable of success "not yet sure if this team is for real"
To be fair, that dictionary also gives genuine as one definition of just plain "real", but I do think that "for real" is much more about legitimacy than "real", to the point that there is a definite difference in meaning. Consider the following examples (mine, this time):
A: I don't think this pizza is real.
B: I don't think this pizza is for real.
If I were looking at a plastic replica of a pizza, I could say A but not B. In contrast, if I had just been given a pizza that I was told was from a famous restaurant, but in fact looked very unappetizing, I could say B but not A.
Of course I contrived that example to demonstrate the difference in meaning. To return to your original question, Heaven is real and Heaven is for real do indeed mean pretty much the same thing. But my point was to show that real and for real are not always interchangeable.
As for the grammar part of your question, I don't think for real is grammatically remarkable - there are other structures of the form "[Noun] is for [adjective]", e.g. This coffee is for free.
Best Answer
It can be used as either a prepositon or an adjective.
Above used as a preposition takes the following form: noun + verb + preposition + noun (e.g., ceilings are above floors, the plane flew above the clouds, I reached above me, etc.). This is its most common usage.
Above can appear, as an adjective, in the prepostitional phrase of an imperitave and tends to modify written text: command + prepositional phrase (e.g., meet at the above address, refer to the above text, call the above phone number, etc.). In each of these cases, however, it is implied that the noun being modified by the adjective, above, (i.e., address, text, phone number) is written above wherever the subject of the imperitave (i.e., the reader) reads the command. In other words, above is never non-prepositional.