MNIONC, I would like to chime in here and give you my perspective as a woman who was an active believing JW married to a fading husband. IMHO your wifes attitude toward you is even more troubling than the elder who is butting in.
#1: When my husband stopped the "family study" I simply studied with the kids MYSELF. Why oh why would she entrust her sons everlasting life (from the JW perspective) to anyone else??????? This smells fishy. What does the elders wife think about this unusual involvement between her husband and your wife and son? Either you or your wife should conduct this study PERIOD. You decide.
#2 You teach people how to treat you. Is this how you want to live? With your wife living independently of you? Why be married then? You don't have to be an ass!@#$ to do this. She is acting like a rebellious teenager. You need to require better of her. State clearly to her what is not acceptable. If she defies you then there will be consequences. Usually tightening the purse strings works well. Whatever you decide will be the consequence, calmly and firmly STICK TO IT. Let your yes mean yes and your no mean no. Believe me she will respect you so much more. Like a bratty teenager she will fight and yell at first. You must ignore this and leave the room. If she doesn't improve over time, I would seriously question your marriage.