Well, I can't argue with you much on what was dumbed down in the translation of the book to the movie.
I can argue with you about your concern for spoilers... on a movie released 9 years ago. :-)
As far as running the Machine twice, it wouldn't have worked. But that was only explained in the book.
I do recommend reading the book, if only for the epilogue. Those few pages still send shivers down my spine.
There were some other little bits of the movie I liked. The 3-D nature of the message and the location of the primer. Also the V-TOL aircraft, the Japanese techs at the launch site, and the graphics of the Machine itself. What Sagan had wouldn't have worked on film.
Re: the 18 hour thing. It could be that Ellie finally figured out what here role was supposed to be and decided to play along. Becoming a more political animal, if you will, with Kitz (the James Woods character) in the lead. He knew exactly what had to happen, and how it all had to be played.
I suppose I'll probably get around to reading Contact at some point. Incidentally, it dawned on me today that the fact that this movie was based on a book explained a couple of minor curiosities relating to "why Vega?"—because that places the events in the late 1980s, so the distance makes more sense, and because they weren't calling people "vegans" yet then. :)