Any deviations should be verified with Van’s.
As mentioned in section 5, regarding TE double-flush riveting, the shop head will not fill the countersink.
The 3.5 rivets for the trim pushrod are enough to keep the three pieces together. Plus, the pushrod is further kept together by the attach points on the ends to the servo and horn.
The problem I see with longer rivets is that you are potentially over-sizing holes that are already very near the edge of the part, and the material around the rivets is being deformed more than it should.
I am not a AE. Just my 2c.