You may want to go a bit longer, depending on how she sleeps.
I personally like my quilts to be a touch on the large side, so that I can tuck them under me. For instance, my quilts are 75" long and 50" at the widest for me at 5'10", 150lbs, and a side sleeper.
Also, with your Golite quilt, it has the footbox built in. When you close up the foot of an open quilt, like it sounds like you want to make, you will lose a bit of the length of the quilt.
Be sure in your calculations that you note M90 has a 58" width, so won't be quite long enough for your purposes, I think. The 2nd nylon at DIY is usually about 65" though, I believe.
I'd also make the suggestion of using both velcro along with one snap at the top. I've tried both alone and neither worked well. I did recently make a quilt that just had loops that I tied shockcord though, and I like that as well, but it is a summer quilt, so a small draft is fine.
I haven't done any sketches on this yet, but hopefully some of this helps just as general thoughts.