I would say: if you're comfortable wearing it and the excess seam allowance doesn't ruck or otherwise bother you - leave it! Apart from anything else, why put it back in your mending pile?
Alternatively, you could trim the fabric and fold a small hem and fix it with something like blanket stitch, so the raw edge is caught within the stitch. If you had an ordinary sewing machine, I would use zigzag instead of the blanket stitch or two rows of straight stitches on the hem.
Maybe there is some way to seal the edges by smoothing a thin layer of a flexible adhesive along it and allowing it to dry (do you have copydex)?