Iuvenalis,
As for Bishop Thuc being mentally ill, someone just saying it does not make it so, even if they are a holy priest or bishop. Especially if they had no current/recent contact with him.
I recommend anyone who doubts his ability to ordain/consecrate validly, that you look at the thucbishops.com website already mentioned for one. It recently has a new posting on the talk given by Mario Derkson on this subject in Spokane. There is an easy to follow chart at the site as well if you want the basics to go through.
From what he has written and said, Bp Kelly doubts the Thuc consecrations. Unless someone can show differently, Bp Kelly never met the man. While we do not have to meet someone to necessarily know whether they are sane or not(other people can tell us about others), there are people who knew Bp Thuc that are still very much amongst us.
One of these people is Fr Francis Miller, OFM of Christ the King Chapel in Lafayette, LA (337)-261-1225. Fr had the privilege of living with His Excellency for about of year after the 81' consecrations when the bishop was staying at the Monastery in Rochester, NY. He served his Mass, brought him to doctor appointments, and interacted heavily with him. If you want to get the other side from someone who actually knew him, not just about him, do yourself a favor and give him a buzz.
It could help you or your loved ones. Fr is easy to talk to and I believe someone who is objective and truly searching for the truth can stand to learn from what he has to present. If you don't accept them right now because of doubts, fine, but it could be that you have not had the information presented to you in a reliable, cogent manner. It is the intellect here that should rule. Not the heart, nor just "feeling."
I just thought I would add this. Do look into it. Please pray for me and I will pray for you.
Joe