Let me preface what I am about to say with two things. First, I understand the spirit of what you're saying; I also believe the short ending of Mark is, at best, odd. Second, I believe in the longer ending of Mark precisely because of the preponderance of widespread and ancient witnesses to it, so this isn't because I disagree with your acceptance of this text.
That being said, is it reasonable—is it even confessional or biblical—to say something like this: "If all the variants in the world said likewise, I would not believe it"? This seems rather extreme. Is this not an instance of doing textual criticism on the sole basis of what we believe the text should be a priori? If there were no evidence in all the world that a particular passage was genuine—if there were no manuscript in existence that contained the given passage, and if there were no historical records of there ever being such a witness—would not WCF 1.8 demand that we reject its authenticity, seeing that it is clearly not preserved? I ask this not to criticize or play "gotcha" but rather because this sentiment seems to go beyond the boundaries of reason, confession, and the Scripture itself.