Second, the Fathers are clear that those who refuse to accept the definitions of faith made by a general council are to be excluded from the Church as heretics.

You're conflating the term de fide with infallibility

But while there could theoretically be some small mistakes, for an Ecuмenical Council to teach heresy or even grave substantial error to the Universal Church? 

There are many teachings of the Church that fall short of being de fide. 

Things can be taught infallibly even if they are not de fide. 

Secondly, you're confusing indefectibility/infallible safety with infallibility in the strict sense.

quite another for an Ecuмenical Council to teach HERESY to the Church, as you have claimed.

I disagree.  The problem with sedevacatism and sedeprivationism is that they lead to theological and philosophical teachings that overturn dogma.  
 
Why cannot an fallible council approved by a pope, churchmen teaching by their grace of state, teach heresy? The reply is typically that the Indefectibility of the Church would not permit this.  But here is the problem.  The Attribute of Indefectibility has not been dogmatically defined as has been the Attribute of Infallibility.  Much of what is believed concerning this Attribute of the Church is the product of theological speculation and Catholics are free to speculate how this Attribute is exercised and preserved in the Church.  
 
Dogma establishes the limits of theological speculation and as long as a Catholic does not offer any conclusions that oppose revealed truth, he is free to consider other possible explanations.  It is from theological speculation that we have the common opinion the Indefectibility serves as a personal non-infallible infallibility of the pope protecting him from error in doctrine and morals in the exercise of the authentic ordinary magisterium based upon his grace of state.  This theory has a number of problems that are not just evident since Vatican II but can be seen throughout difficult times in Church history.
 
We know that the Attributes of the Church are powers given to her by her founder, Jesus Christ, that enable the Church to do specific things.  But just as in man, where each individual sense power has its specific mode of operation and individual ends but still has considerable overlapping with other sense powers in many general perceptions, so do the powers of the Church. If each power is considered with respect to its individual end, they correspond to the three principles duties that God has imposed upon His Church: to teach, to worship God and sanctify the faithful, and to govern specifically enumerated by St. Pius X in Pascendi.  These duties are possible through the powers of Church given to her by God, that is, Infallibility, Indefectibility, and Authority. It is important to remember that these Attributes are firstly Attributes of God and only Attributes of the Church because the Church is a divine institution.  The powers resided primarily and essentially in the Church.  They resided in churchmen only secondarily and accidentally.
 
The specific end of Indefectibility is to worship God and sanctify the faithful.  Common theological opinion holds that Indefectibility of the Church means that a council and pope could never impose doctrinal or moral error on the Church.  This leads to conservative Catholics. like Emmett O'Regan. who believe that the pope possess a personal never-failing faith and Indefectibility means there is no possibility of error from the Vatican II or concilar popes therefore we must accept them and all they teach.  It also leads to sedevacantism/sedeprivationism that agree in general principle with conservatives but therefore conclude that the pope cannot be the pope to preserve the Attribute of Indefectibility.  I contend that both of these conclusions are wrong and both lead to overturning of dogma.
 
If you consider Indefectibility as primarily an Attribute of the Church in light of the specific end of this power, that is, the worship of God and the sanctification of the faithful, these ends have never been absent from the Church since Vatican II. Just as Noah building the Ark condemned a sinful world, so Catholics faithful to tradition and the "received and approved rites customarily used in the solemn administration of the sacraments" condemn the conciliar Church. It is traditional Catholics that will not betray the faith that constitute the evidence of the Church's Indefectibility.  
 
This theory may not be correct but it does not overturn any Catholic dogma.
 
As far as the exercise of Authority, it is strictly addressed in Catholic moral theology.  The proper response to Authority is Obedience.  But the ultimate Authority is God and all Catholics are obligated firstly to obey God.  There are about a dozen subsidiary virtues under the virtue of Justice.  These subsidiary virtues are hierarchically related.  The first and most important virtue under Justice is the virtue of Religion. This virtue primarily concerns giving to God the things that are God's and typically can be quantified by specific acts.  It is the virtue of Religion that governs obedience. Obedience is only a virtue when it is properly regulated by the virtue of Religion.  When it is not, any act of obedience is sinful.  There has hardly been any imposition of Authority since Vatican II that does not directly offend the virtue of Religion and must therefore be opposed.  
 
R & R does no damage whatsoever to Catholic dogma or Catholic morality. Two things are necessary for any reconsideration: firstly, a conciliar pope will have to directly engage the Attributes of Infallibility and Authority to bind the Church to doctrinal and moral error, secondly, sedevacantists/sedeprivationists will have to produce a pope who is generally accepted by Catholics faithful to tradition.  
 
I do not think either one is going to happen.

Lastly, every faithful Catholic should remember that the two greatest tests by God, the angelic test in heaven and the person of Jesus Christ to the Jєωs, required His chosen faithful to reject the constituted authority established by God.  It should not surprise anyone if this should happen again.

Drew

"If you consider Indefectibility as primarily an Attribute of the Church in light of the specific end of this power, that is, the worship of God and the sanctification of the faithful, these ends have never been absent from the Church since Vatican II."

and

"Two things are necessary for any reconsideration: firstly, a conciliar pope will have to directly engage the Attributes of Infallibility and Authority to bind the Church to doctrinal and moral error, secondly, sedevacantists/sedeprivationists will have to produce a pope who is generally accepted by Catholics faithful to tradition.  

I do not think either one is going to happen."

Just as Noah building the Ark condemned a sinful world, so Catholics faithful to tradition and the "received and approved rites customarily used in the solemn administration of the sacraments" condemn the conciliar Church. It is traditional Catholics that will not betray the faith that constitute the evidence of the Church's Indefectibility. 

I wonder what is your understanding of the dogma that there is no salvation without personal submission to the Pope of Rome, Mr. Drew.  It is a defined dogma of the Catholic Church that no one can be saved who is not subject to that flesh and blood Vicar of Jesus, the Roman Pontiff. How can you recognize, in good conscience, who the Pope of Rome is, and still persist in severing communion from him?