Jonathan, it’s hard for me to discuss such topics in another language, but I discoved two things:
Your creation of the columns-guides is wrong. If you stay with 12 columns, you will not have the possibility of autoflowing text. So I recommend working with ruler-guides on an additional layer and go with 3 columns and if needed, with different masterpages.
What you are tryin’ to archive cannot work! Your grid is based on 6 pt, but the placeholderframe has 72 pt. This sounds good at a first glance, but you forgot, that the first line has no leading. So, in my opinion this 1pt differences between e.g. “J” and the top of the frame is not a problem, but this 6 pt at the bottom is.
So, there are always two rules:
1. Use whenever possible a grid
2. Know when you can or should make some overrides to the grid
Said, you will have a perfect looking result if you go with about 67 pt for the frame and move the textframe a bit to the left. Note also, that life would be easier, if not everything is a single frame. I would do anything in one frame and anchor later the images.
So far
Kai