I don't know how you would confirm whether or not there is another piece of glass in there or not. It doesn't have to be huge -- could be a tiny thing just in the wrong place. If you've ever had a glass sliver in just the wrong place yourself, you'd understand.
I think I would look at having a vet come out as well. Perhaps the vet would be able to flush out the initial cut area and wash out anything that might be there. However, after a whole week, that may involve some cutting to open up the area again. :( That's all I can think of. Sorry.
After a week with no improvement, I think your horse needs a helping hand. Good luck.