1. My Pentax V reads 12.6 with a set of batteries that have been in for a couple of years.
2. I zero to the center of the black dot and I get realistic readings.
3. The scale in the Pentax is truly EV when the ISO is set to 100. Then, a gray card reading in bright sun will read 15 on the scale and when the exposure dial is set to 100 you will get an exposure of f/16 @ 125. I do find, however, that the Pentax suggests about 2/3 stop more exposure than my Gossens.
4. I doubt that my spotmeter has ever been calibrated; I am the second owner and I have owned it for 10 years and never calibrated it and it is still spot on.

I am also interested in which Gossen you are comparing to.