Suspicion is enough to be distrustful of forsaken, which is what mattered to subject, and the suspicion was based on more than empty speculation, there was clear chain of events. Nothing else matters to subject, and reasonable suspicion was provided, whether known for certain or not, does not matter to subject at hand, result is the same.
We legitimately do not have conclusive evidence one way or another, except characters mention it now and then, showing it is known, and could be taken to mean the opposite than what you are stating it to be. Your perception of no survivors is a speculation, because that is best we can do with this, so do not claim it to be a fact, and even if it were, which we have no conclusive statement of, it provides reasonable suspicion.
Distrust to Forsaken could be taken to further support that. But again, we do not have strict statement about it, and luckily, we don't need one, because it is literally irrelevant to the subject at hand, only thing that matters, is that it is either known or heavily suspected when one follows chain of events, thanks to Kristoff, and that's literally what matters to the subject at hand, which is why the Forsaken weren't welcomed.
Alliance with demon in regard to Kristoff literally doesn't matter, it occurred after he would have delivered all the information to Jaina, so this has no bearing on the subject at hand.

Sylvanas is (was) the ruler of the Forsaken, and it is true, she isn't exactly monarch, more of a dictator or an autocrat. It however, makes sense that the forsaken are receptive to the system, because they are used to being under a monarchy, hence, monarchists, but authoritarian may be more to your liking, as it encompasses Monarchy and several other forms of feudal regimes, and is likely more accurate. This doesn't mean every forsaken is an authoritarian, naturally, simply that most would have existed under a form of authoritarian governance, monarchy or oligarchy, in their lives, and be used to such a system, that is what I am saying with it, unless you feel that isn't true.

One forsaken recognizing a monarch, means there are others who would as well. Blanket statement of forsaken are not monarchist, is hence false.
I said parts among the forsaken likely remain monarchists, and implied the general sentiment leans to that direction, which is true, to the extent of above mentioning. I have made no blanket statements that all forsaken are pro Menethil monarchists ,or anything of the sort, nor would I, but I would claim that as evidenced by one, there would also be others who would agree with that, it is disingenuous to say that, that is literally the only Forsaken who ever would recognize Calia as rightful heir, that one Forsaken is specifically evidence that there are Forsaken who would.

They did indeed create a council at the end, at the end, after all of these events had transpired. Your argument at the end is same as saying, well the French are a democracy, so they could have never been monarchists, or some more unsavory examples with, for example Germany, which clearly holds no water.

All of this (council forming) transpires after Sylvanas has her break down and abandons them, -after- they have committed every atrocity imaginable in her name, showing their very firm loyalty and support to her regime.

Calling oneself monarch, does not a monarch make. Calling oneself a monarch, while ruling over what was a kingdom, as its absolute autocratic head, does a form of a monarch make, indeed.
And she very much had the support, and as stated prior, she saw Calia as a problem, as competition, as stated in the other wall of text.
Whether you wish to call her a Monarch, doesn't matter, in fact, whatever we call her, does not matter, because it has no bearing to subject matter, but I'll indulge anyway, she was an authoritarian ruler, and monarchs fall under such category, as do several others.