As much as i agree with you, Cletus, and I absolutely do, what you say comes across a bit harsh, personal or defensive. It's extremely sad that there are traditional Catholics on all sides of the fence that passionately believe what they believe, in good conscience, even though you may not conceive how those who give tacit authority to the "pope" by the use of his title do so.
What mystifies me a little is that in regards to something so incredibly particular as the theology involved in coming to the sedevacantist conclusion (or the SSPX conclusion - although I do have to say it seems to me that many who adhere to that conclusion haven't really, really researched it) one wouldn't be more analytical in attempting to explain it to someone who won't be convinced by a (albeit passionate and well-intentioned) tirade. What is the cliche? It's easier to draw flys with honey than with vinegar?
I don't mean to attack you, especially as i agree with your points so thoroughly, and I hope you don't take it so, but I personally think that one of the best things, one of the greatest advantages about discussing things of the faith when all of us are trying to be true traditional Catholics, is that we can do so without letting our emotions color what we discuss.
Of course we are all dismayed at the situation in the Church, and rightfully so, but it's hard to pay attention to what someone is saying when they are yelling it at you so to speak. A friend of mine once said that he always has a hard time converting people because his inclination is to hit them on the head with the hammer of truth, and, unfortunately, like a mule, too often times people dig in and don't even care to listen to what is being said because of it.
I don't think that dulcamara disagrees with you more obstinately because you are pointing out her "heresy" more (in quotations b/c even though I am sedevacantist I cannot blame them for what they do - there has been no valid pope to say which side is right and which is wrong) but because no one likes to be attacked on what they really and truly believe. It is one thing to hold a discussion, another to rant at someone and then discharge them as a heretic and going to hell because they didn't listen to you.