I agree with just about all your criticisms (though I didn't have a problem with Machlachlan's age, I know it's not in keeping with the book but I'm not sure any actual teenage actor could have pulled the role off).
I'll also add that if Lynch was only allowed to work with a 136-minute running time, he did a really poor job of choosing what to keep and what to excise. Paul and his mother literally don't join the Fremen until 90 minutes into the film, and there's only 45 minutes left after that! The entire final battle of Return of the Jedi took nearly that much screen time on its own!
One of the things that annoy me most in defense of this movie are those who claim this film had good f/x for its time. The Empire Strikes Back, Return of the Jedi, Star Trek II, and Star Trek III all came out before this film and all had far superior special effects, and they were all made on lower budgets.