The only one you can possibly get at is the rivnut hole closest to the larger access hole. You can get a bolt from the inside of the frame and reach it with a box end. You cannot reach any of the others that way and you are not going through both sides of the frameTo the ppl that are just using long bolts that go thru the frame. Probably won't make a different but you are going thru a hollow frame and puting clamping forces on both sides of the frame, you probably will be collapsing the frame a bit when you tighten them down. In most cases that you do this it should really have a sleeve that keeps it from doing this.
I tried to use the bolt and nut provided for the rivnut but it didn't workout well for me, probably just user error. I found the the bolt and slightly too big nut with the washers trick worked better.