The coroner stated that suicide was not his final conclusion. He had put suicide on the report, but it seemed like he was still investigating it? Or is he just saying that? Nonetheless, there is some pretty compelling evidence against Millard, most notably how he lied his way through the police interview. He said he was unaware of any guns in the house, yet the murder weapon was proved to have been acquired by him and had his DNA on it. Undeniable. He said he spent the night at Smich's, which has proven to be a lie as well. Leaving his phone and credit card shows that he was setting up an alibi. His text to Smich after his police interview "I gave them your name, but not your contact info" is confirmation that Smich was already on board to confirm his alibi. Phone pings have put Millard in the area of Maplegate at the exact time of the murder. Beyond that, we have already seen evidence of a motive, Millards fear of WM blowing his inheritance.
Bottom line is that the coroner stating it might have been a suicide is irrelevant as a bullet in the head is a bullet in the head, whether its murder or suicide. I honestly don't think that in this case there would be any way for the coroner to distinguish between the two.