ok I'll take a stab at it, first if it leaks more now than before the valve adjustment, I'd make sure no vacuum lines are pinched under the valve cover. If you had to retighten it three times it may be cut in half by now.
Next fix the things you know are leaking it will make the others easier to find. As to using oil are you adding a quart to bring it up to the second mark? On my 617 it seems to use down to just above the lower mark and stays there, if I "top" it off it uses down the same spot and stops using. Black soot on the back of the car-- yes its a diesel, if you have leaks in the exhaust, combined with sustained highway speeds this would be normal, all the oil leaks will add to this. As to the mpg I cant really say what the norm for your car is, on my '85 300 turbo I was get about 23-24 after bumping up my tire pressure, adjusting the valves and fixing a dragging emergency brake and a stuck caliper, I'm up to 25-26.
also the 10 or spots you see on the floor could be from two or three leaks, it does blow around in there a bit Good Luck, James