Frenchy,
You asked:" What should be done about about serious wrongdoers ( thieves, drunkards, drug users and pushers, fornicators, child molesters ect.) in the congregation?
Most of the serious wrongs you mentioned can be handled by the law of the land in various degrees of punishment from serious to more serious. Even fornication such as sleeping with someone under age, and depending on the country you live such as in some Arab lands, fornication is illegal with various degrees of punishment by the arm of the state.
So the first step would be to let the law of the land handle it.
I personally believe the society is wrong in their approach to the system of justice they have set up by the abuse of the disfellowshipping arrangement along with the by-products of shunning.
The spirit should be to gain your brother or sister or child as was Christ's spirit with the fellowship he provided with the arrangement of the christian congregation.
But you have to remember that doesn't mean to be dumb and naive as far as serious wrondoers are concerned due to the corrupting and dividing influence it could have on a group of people, first let the law of the land deal with them and second if the christian congregation has to deal with the odd problem, then great care should be made to establish the facts in cases of fornication or other, and great wisdom should be used in difficult case where many peoples lives are involved. If guilt is established and I'm talking very general here, I would give someone in the congregation 30 days off from the congregation and they would be welcomed back at the end of this period of time. All of this would be done without attaching a label to the person such as disfellowship, excommunicated or call it what you may. In this way you would make it easier on the individual to see his wrong way and come back without that label attached to him, to associate within the brotherhood and especially with his immediate family who could provide him with more support than anyone else in some cases to do the right thing before Jehovah God.
It is also everyones responsibilty as Christ encourages us to choose our close associates in a manner and hope that they too will manifest the fruits of the spirit. All of this can be done discreetly without showing yourself a bigot.
LOVE CONQUERS!