I have to ask the double stupid questions.
1. Doesn't decay do half the hit points damage to the minion up each hit no matter what the level trained?
Only vs opponents without AMF. If the opponent has AMF you'll get 100% backlash with a base decay if not using NSCs, and even with NSCs it wont do half damage but you'll still get backlash (Just like any other DD spell). Hence the reason why I like to train my decays.
Quote from the Wiki: "AMF When used versus Decay is based upon the AMF % multiplied by 1/2 of the opponent's Hit Points at the time of casting."
2. Would not the only damage increase be to the minion using decay from anti magic field because of the ag. Unless i missed this minion is junction-ed to a familiar.
I'm kind of confused what you are asking here. Could you clarify?
In viperboy's case his AGs were way too big and it canceled out decays effect and made it 0 which converts 100% AMF backlash damage to his own minion. No familiar was junctioned, and even if there was the familiar is just treated like another minion.
could just be my low level of understanding game mechanics.
Decay's mechanic takes a lot of getting use to, along with a few other mechanics, but everything comes with time.