With all due respect, I would suggest that, because so much of karate history is verbal and anecdotal, the many tales of Chiang Nam, Channan, etc can't be completely discounted. There really is no definitive answer. Most versions agree that Itosu Sensei created the Pinan kata for instruction in the schools. Whether he worked them from another (or multiple) older forms - whatever the name - or created them from whole cloth himself, I don't suppose we will ever know. There are certainly references from several early 20th century Okinawan instructors' accounts to some rather vague, perhaps Chinese form of a similar name. That Itosu stripped the more dangerous techniques from the older form when creating the Pinan is another meme which cannot be proven or disproven, but if you accept that, it seems to agree with an older form as donor.
I never experienced a shotokan instructor running the Heian forms as one, although I have seen Tang Soo Do teachers do it that way. I agree that each Heian form addresses a different subject, and that, at least to me, they don't really feel like one truncated form. (I think Sam Dan the really odd duckling compared to the others) And, of course, there is the legend that Funakoshi switched numbers 1 and 2, which would seem to argue against the flow of 1 through 5 (assuming a shotokan progression of 1 through 5 instead of 2,1...5.).
But an interesting discussion. Personally, I don't like running the Heian (Pyung Ahn) forms together. But I do often do all five in sequence as a warm-up when training alone, just to get the body working.
By the way, there is a book by Schmeisser about his efforts to reconstruct two Channan kata. Link is
http://www.amazon.com/Channan-Heians-Elmar-T-Schmeisser/dp/1412013577
I have it but confess to some confusion, and decided to re-read it some other time. As I recall, he really is theorizing about the kata and the techniques (as opposed to some discovery, say, of the original kata).
