It's a cute quilt. In my opinion, you can't press curves like this into submission. When you complete the quilt and wash it, it will go awry again. Ask me how I know...
Seam rip out every other column of the quilt and sew it back on from the opposite end of the column. That should reduce the curving significantly.
Personally, I would probably hate all that 'undo' and 're-do', but have found that fixing a mistake like this firmly embeds a loathing for that mistake in my psyche, so I am very careful NOT to make the mistake again.