You sound as though they have already experienced defensiveness and hostility.... Sorry if I'm mis-reading your post.
My husband's involvement in clinical trials (now in remission for Stage IV lymphoma and loving every day) at the U of Chicago showed us that the doctors LOVED to have the most technical questions since it showed we were (are) pro-active, involved and actively invested in treatment. We brought our list of questions to his "team" and you could see them rev up in anticipation of answering. This is "their thing". This is what they love to do. The clinical trial means they are considered so competent in their field that big pharma trusts them to know their shit. And they do. Treat them with respect but pull out every question you have. In our experience you will instantly win their respect. Our doctor was/is one of the best in his field for lymphoma treatment which meant we gave him no mercy on our questions. And I think he loved (loves?) us for it.
When my husband would sit in the chemo chair, and we were waiting for the doc to come by to check on him, we noticed that the entire "team" - interns and all - came to his check up. Nobody else in the entire chemo treatment room (a wide open space at U of C) ever got nearly the same level of interest and involvement.
We loved it and I suspect they loved it judging by the way everyone acted. It was fantastic. When my husband went back for his 1 year check up, the doc knew us - really knew us based upon personal conversations we'd had that wouldn't have been included in his file. And I know that's because we were so damn inquisitive.
As is typical in most situations, start out with a compliment. I know it's hard with the relapse not to feel angry but you already know your dad has a tough genetic lottery number. If you are open and truly only interested in seeking answers - and not nailing his ass to the wall - then, imho, fire away.
Good luck. We came away from our sessions with the doc(s) empowered. Although the news was never great, at least we always came away better informed. That's never a bad thing when making tough decisions.
Peace and hugs all around during your difficult times.