Looks like a great project, and I am thoroughly enjoying the build along, thanks for putting it up. In my humble opinion, I would get the core bent to shape before proceeding with any more layout, as the centerline might change after bending. Then I would draw the centerline as Mike suggested, on a flat surface with a spacer block, and do the layout off that. Also I would bend it to shape as thick as the thickest part of the final bow, this will help minimize twist during the horn glue up.
I wouldn't shy away from going for 58" ttt if stability is the main concern. I've naver made a horn bow longer than 48" ntn, and I am comfortable stringing most of my bows in the field and dragging them through the woods. Once they are tillered and tamed, their behavior becomes quite predictable (unless, perhaps, they are left unused for months, then they may begin to revert to their old ways). I almost always use the korean stringing methods, both sitting and standing variations, the standing method is easier with longer bows.
I look forward to seeing your progress!