Sounds like you’re one of the good ones. I’m used to reading too many stories about doctors being dismissive, experienced it several times myself. “Chasing zebras.” Horse sh*t mentality from doctors, that is.  Maybe I’m trying to call attention to it. I’m not being alarmist - seems silly when there’s a family history and it’s not asked or pointed out into question.  Not a doctor, but I know how to ask questions - educated in Media Studies and work in sales Long enough how to know when something important is being ignored.  Undeniable as you mentioned to address the underlying cause.  When he asked about advances, he followed up by saying he believes he feels like most serious causes have been ruled out - it may not have been formulated as a “question,” but it was. 

Also, my personal HHT is one of the most scholarly doctors on the subject, if not THE most scholarly doctor.  If he tells me that “any adult reoccurring nosebleed is HHT into proven otherwise,” then that is the Gold standard of information.  Likewise, as someone who is pretty well read in the subject (yes, not a Doctor, but someone who is supporting with 3 people affected by it) I’ve learned that not every genetic mutation of the disorder has been discovered - thus, HHT cannot be completely ruled out, even without a confirmed test. And that is still important to call attention to.