Answer: You are right, it is very difficult to make a valid guess based on your description. I think you might want to consider taking a look at a book or two about Japanese maples to see if you can identify it that way. One I would recommend is the well illustrated Japanese Maples by J.D. Vertrees and Peter Gregory from Timber Press, ISBN 0881925012. It is available in book stores or perhaps your library would have it.
I am also wondering if there is any chance that the tree was grafted and has possibly had some incompatibility between the top and the root stock. By that, I mean it is sometimes possible for the fancy grafted top to be somewhat overwhelmed by growth coming from below the graft line, either from the trunk or from below ground. If you look carefully at the tree, you might be able to tell if this is the case -- you would see a separate branching system attributable to the green leaves and one for the red leaves. It would be unusual, but not altogether impossible.
I'm sorry I can't give you a more specific answer.
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