I followed your lead and cut the dress yesterday. It is much better shorter, but I am still not completely pleased. I will try to insert some darts at the back to give it a more defined waistline.
And with the sleeve/straps... it is funny how we sometimes do completely wrong things believing all is well. I had to laugh so hard about myself, when I noticed I put the sleeve inside out.
I hope you could rescue your backpack and its straps :-)