Since comments are not exactly pouring in, I will just tell you the reason I think the Russian "kheppi end", borrowed from English "happy end," is weird. It's because, at least in my dialect of English, one rarely says "happy end;" it's much more common to talk about happy endings. So it's odd that Russian didn't borrow that form. Seems relevant that end and ending mean almost the same thing; that they don't mean exactly the same thing (and they don't, I think) is a fine enough point that it doesn't interfere with understanding.
But how and why did that –ing get lost? Did the common appearance of the words (written) "THE END" (not "THE ENDING") at the end of films, stories and plays have some influence on it? Or maybe it was influenced by the relative cognitive difficulties of dealing with nouns that have verbal morphology on them? (I know, I should know what they're called. Gerundives? Anyway, Russian doesn't have them.)
Or is this just another of the many lexical differences between British and American English that have made me look dumb so many times in the last two years? (Seriously. You have no idea how many there are until you go about correcting what you perceive to be errors in Russians' English, only to find out that they're actually speaking correct British English.)