Do you mean the seams that are in each block corner- connecting one strip to the other? If so,if it were me, I would undo those blocks, and resew those seams---- either your stitch size is too large- go down to 1.6, or your final strip in the block is way to small...
when doing a strip block it is good to try to keep that last strip wide so it's not just a crumb on the corner.
I don't know about using interfacing or appliqué over undone stitches...never did that. Especially if that is for someone else, I would want it as good as I could do it.