They needed an ending that was big and melodramatic and action packed, and that's what they came up with. The movie was successful and entertaining to its target audience. So, who are we to say?

The story is king.

I am aware of the Super 8 film connotation, thanks. It was originally a double entendre and I can't do anything if you're not prepared to accept that, but it happens to be true.
"I don't reckon I got no reason to kill nobody."

No, the reason it was called Super 8 was because of Super 8mm film. They talk about this in the featurettes.

It was actually called Super 8 because there were eight kids: Joe, Charles, Cary, Preston, Martin, Jen, Alice and Donny. Although that was very subtle and a feature of previous script drafts that was largely lost in the final draft.
And, of course this was a love note to early Spielberg, they never tried to pass it off as entirely original. They didn't hide that fact, even putting Spielberg's name higher on the poster than Abrams's.
And I'll give you that the interaction between the boy and the alien was hammily set up, with the script giving a couple of references earlier in the script that 'when it touches you, you know' and the alien's eyes morphing into those of Joe's mother. I can see what they were going for there, and they didn't quite sell it but I don't think it's fair to say on that basis that act III was entirely lost, that was one scene that didn't quite come out how it was intended.
Particularly loved the trailer, with the Cocoon overscore which in many ways was better music than Michael Giacchino's.
"I don't reckon I got no reason to kill nobody."

It's called super 8. What does the creature they captured on super 8 film have to do with anything? Weird splitting up of characters. An ET/Close Encounters rip moment. Strange scene with boy and monster where he reasons with the monster? I don't buy it.
One of the best trailers I've ever seen though. lol

I don't follow. How did it fall apart? What were you expecting to happen?
"I don't reckon I got no reason to kill nobody."