As others have mentioned, you need the slides to evaluate the scans.
They sent you jpgs, which do not have the color information that a Tiff or Psd file would have, so even with adjustments, you're swimming upstream. (think "sow's ear)
I did a screen capture of one of your posts and did some simple adjustments with curves (much like what was suggested with levels, but I prefer using curves, same adjustment capabilities and more), then a hue saturation adjustment. It looks better but not great. Also, the film is grainy, which doesn't help.