This leads me to a writing a post in which we can look at the article in detail, word-by-word if you will, and then war-game it to speculate what could happen in such an event should it occur.
The Congress, whenever two thirds of both houses shall deem it necessary, shall propose amendments to this Constitution, or, on the application of the legislatures of two thirds of the several states, shall call a convention for proposing amendments, which, in either case, shall be valid to all intents and purposes, as part of this Constitution, when ratified by the legislatures of three fourths of the several states, or by conventions in three fourths thereof, as the one or the other mode of ratification may be proposed by the Congress; provided that no amendment which may be made prior to the year one thousand eight hundred and eight shall in any manner affect the first and fourth clauses in the ninth section of the first article; and that no state, without its consent, shall be deprived of its equal suffrage in the Senate.[ emphasis added]
In order for a convention to take place, it must follow the Constitution itself. It cannot deviate from it, or it can be declared illegal, and its delegates may be arrested and charged. Certainly, an attempt to overthrow the Constitution cannot be entertained, unless that is the intent. If such was the intent, why bother with legal technicalities?
Thus, two thirds of the states must apply to Congress for such a convention. The Congress can choose the means of ratification. Consequently, the Congress cannot initiate it, but can choose the means by which any changes can be ratified, amongst them being only two means from which to choose.
If there were a "runaway Convention", whatever they produce must be ratified by the means that Congress specified. If a rogue Convention proposes any other means, this reverts back to sedition, and they could be arrested and charged for that. Does anyone believe that the US Government is going to idly stand by and watch its authority being stripped away, and not offer any resistance to this whatever? Any deviation would at least be stopped in the courts, and in short order. I'd say that whatever comes out the convention will be challenged to the hilt, as any attempt to rein in the power of the current government is not likely to go unnoticed, and their attitude towards this is not likely to be indifferent.
Three fourths of the states must ratify. No other way of ratification can be legal. If there is any attempt to implement changes by any other means, it can be stopped, and most likely will be stopped. It is highly unlikely that anything revolutionary is likely to come out of the process.
The states will have control over the selection of delegates. Reliable people are likely to be chosen. If they turn out not to be, then the results will not likely be accepted ( see above).
In my opinion, a runaway convention is unlikely, and even if it did occur, it won't achieve much, if anything at all.
What if the worst occurs, and a runaway convention completely rewrites and changes the Constitution, and nobody did anything to stop it? Then I would submit that this is already happening anyway. The assumption is that we have something to lose. Actually, we have nothing to lose, for if we do nothing, the Constitution will be usurped anyway.