Interesting. We had a pair of beagles that we got for my sons when they were young. The dogs were brother and sister.
The female had an insatiable appetitie (up on the kitchen table if ya left a chair out so she could get up, loaves of bread in plastic wrap right off the countertop, butter, etc. etc.) She would eat wild rasberries that grow here whenever we took her on walks - would nibble them right off the bush. She also liked to eat clumps of grass that fell off the bottom of the lawnmower deck. She wouldn't eat deer poop (that we ever observed) - but she did love to roll in it. But she would eat her own turds. She just wouldn't ever stop eating anything and everything in sight - she would even 'steal' her brother's food, which would be pretty funny to watch - she would slowly sneak over to his bowl while he was eating and nudge her way in. He would start snarling and showing his teeth but that didn't stop her - she would just continue to edge closer, head down. He usually ended up grabbing her by the scruff of her neck with his teeth. She would stand there, head down. Eventually he would let go and back off - and she would wolf down his food too, after just having eaten her own. She once got into the pantry and pulled out a Tupperware container that we kept the brown sugar in - chewed the lid off and ate almost 3/4's of the sugar (several pounds worth) before we caught her. And she would regularly troll the cat pans for a fresh snack.
She was also very skitish as you describe - but she wanted to right next to you all the time (thunderstorms really freaked her out - she would try and hide, but would never stay in whatever place she picked if the thunder kept up - always had to move to find a safer spot) Her brother was rather aloof in comparision (at least until she died)
Other than possibly having some bodily "condition" (thyroid maybe ?) or a vitamin deficiency, etc., the only thing I can come up with is this - she was the runt of the litter and much smaller than the others (her name was "Bitsy", cause she was just a 'bitsy little thing when we got her) She was spayed also.
There was always someone around too - my wife didn't work outside the home and my parents lived with us at the time.
Sadly, she passed away in the fall of 2003 at the age of 11.
Her brother had been doing pretty well up until this last week - he came back from the groomer a couple of days ago and she commented to my wife that he seemed to be having balance problems. Over a couple of day period he appears to have pretty much lost most of his sight (blank stare, runs into things) and a good bit of his hearing (you talk to him and he often doesn't respond, where before he would respond in some manner any time you said something to him) Probably getting close to time to dig another hole out back.