Nem... Dr. Mulholland (one "L" on the first L, not two) did not accept a paid for trip to the US to give that presentation. He stayed right there in the UK, presumably in his own home. The speaker was supposed to be UCLA neurosurgeon Linda Liau. She was in the UK on vacation, and had intended to fly home, first stopping in New York to give the presentation, when she came down with Covid the day before the event. So Dr. Mulholland, gave the NYSA presentation in her place in the UK via Zoom, or something like that. That's why, if you had even watched the presentation, you'd see that he's not even present at the meeting.
You'll likely find that out soon enough, when he responds to you. But, sympathies aside, I will point out that you did a fair amount of assuming there, by accusing him of taking a paid for trip, that had you done a little more research, you'd not have added that part to your letter. I know you feel that you've been wronged, and perhaps in some manner you have, but I wish you'd would consider that your anger and frustration with the matter may be misdirected.
Medical errors do happen, as do medical diagnoses. But if there truly was evidence of GBM cells found in your biopsy, there are many who would argue that offering you SOC for GBM (given the death sentence that usually comes with it) would be the right thing to do. And, speaking off the cuff, no insult intended, perhaps in your case, it could be argued that it actually may have been the right thing to do. If the GBM present was so small, barely present, throwing all that radiation and chemo at it may have killed any chance it had of growing, and that was the end of it. In which case, you'd actually be quite lucky, despite any issues you've suffered as a result.