I'm not sure any glue would hold if the back pack is going to be used to any account.
I can see your 2nd row of topstitching ... what about doing a 3rd to help hold in place?
For the vertical seam, could you get it into your machine and do an extra row in the seamline?
To protect seams like that, I often use a coverlock type stitch on my machine. I call if faux serging!
It helps to tidy up raw edges and keep them from fraying.
But in the bigger scheme of things, will your DGD or anyone else be inspecting the inners?
Good Luck!!