Steve, I took the liberty to copy and paste your underexposed picture into Photoshop, and with a few tweaks of the Levels the image looks perfectly normal to me, plenty of shadow detail (so not under-exposed) and plenty of mid-tone and highlight detail.
So I suspect it is a scanning problem perhaps, and that you haven't done any post processing? As this a APUG I won't go on about it too much, but you may want to check your histogram in the scanning software and make sure the sliders for shadows and highlights are set at either end of the 'wave', use the middle slider to adjust mid tone brightness. You should aim for a flat sort of scan containing all the information possible that can be adjusted to the final image in Photoshop (or whatever). I can post the image I came up with if you give the say so.
For the spots, well it looks like measles, but as the sofa can't catch measles I'd say bubbles have formed on the negative at some point in the development process.
Steve