Sorry, i am at a bit of a loss here. I really do not understand your report design so I am struggliong with how to help you.
I have tested a version of your formula
replicatestring(chr(13),tonumber({Detail.ExtraLines}))
placing it on a report footer and had no trouble with it expanding over 2 pages when i made it 50 rows.
Your column header suppression question is not making sense to me either. Do you have groups with headers on a group feader level? Are these on a page header? YOu say you have 2 detail rows per page. How are you breaking this with only 2 rows per page? Do you need to suppress the header when both rows have shownav="Y" or either row or some other option?