Re the first issue ... if you're going after jewelry the less discrimination you use the better. Your practice of digging just about everything on the beach is a good one if you're serious about getting jewelry. Not sure exactly what program you used from the book ... it did give some examples where you might use disc in special cases but usually on the beach the less disc the better. So if you were using good search practices, such as, hunting slowly and with good focus, etc and you didn't get jewelry then your coil probably didn't pass over it or it was too deep 8^P.
As for the second issue ... the more experience you have with your detector's response to targets, their sounds, etc the better your hunting will be. However, any target identification system (audio or visual) is designed primarily for simple, straight forward situations. For example, they are calibrated for standard targets (ie coins, etc), which are more or less alone under the coil, and so forth. As soon as you get into the real world and have varying minerlization, more than one target which may be a combination of good and/or bad targets, etc, etc. you should use these features cautiously.
While the sophisicated systems of ID used in modern detectors can be very useful, you shouldn't follow them blindly. I find I tend to use them in special cases (ie when I have limited hunt time, or am tired and losing my focus, etc) rather than as a means of deciding to dig, or not dig, every target. When the response is a real good one, obviously I'm not going to ignore it ;^), if it's a real bad one I probably will ignore it, but if it somewhere inbetween, then other factors come into play (type of sound - smooth better than ragged, depth - deeper than normal may be good, past experience show the site has old targets, etc). I guess what I'm trying to say is to be careful and just don't blindly follow the meter and/or audio ID 8^).
HH ... Gord.