I have a better idea for what I would do. The first movie would still be 1-5+8. That equals 4....wait, got off track. I would change book two to include more action. Make the first rachel book an exciting one. that shows she has two opposing sides to her.
The second movie would be about freeing the Hork-Bajir, and dealing with Aftran and the peace movement. I would include the Chee there, and have Erik be behind the peace movemnt. Like, he started it with some Yeerks he made friends with, and it grew from there.
Third movie would be when things start going south. David comes with the box, and betrays the group. Their cover is blown, and they have to hide their families. It would all lead up to the climax. Auxilaries come in towards the end of this one. I may even have David just be one of the auxilaries, except he goes bad. I may not have the David and auxilary things be two separate cases.
Movie four is the final battle. At the end of the movie, the Anis win. The Auxilaries and peace movement would play a much larger role in the battle. And right at the end, time permitting, I would have the Andalites show up to honestly save the day. except they were too late to help.
Fifth movie covers the Chronicles, with a rewrite of Ellemist Chronicles.
That should cover all the major points. How long each movie would be kind of depends. I would try not to rush things, and would be willing to increase the movie number to prevent that. But I don't think less than four is possible.