Mantra, I'm so, so sorry for the loss of your beloved DH. He sounds, you both sounds, like very special people.
I've read through thread, trying to decide if my input would be helpful. The circumstances around death and the particulars of mental health are very individual and personal that there is no one 'it happens like this and this is what it feels like' but since you are still tormented by the idea that you could have or should have stopped him or seen it coming I though I would add my perspective.
I had post natal depression with my second child that was untreated (I was just on waiting lists the whole time as I didn't present in a way that fitted with their model of who was at risk), after my third child it came back very suddenly and heavily. I knew I wasn't doing well and I had flagged myself up as being at risk but when I become very unwell it was very swift. I woke up early and I 'knew' that I was a terrible mother and doing terrible, untold damage to my children and I ran out of the house. As it was early I didn't want to phone anyone for help (which, in a sane frame of mind, is silly because if they knew my friends would have wanted me to call them), the desperation felt completely uncontainable, I was in so much pain and didn't know how to make it stop. I ran along the side of an dual carriage way and then crossed the bridge over it but stopped about 3/4 of the way along and the thought entered my head that I should jump. I hadn't had any thoughts of killing myself or planned to do so but that thought was so compelling that I felt a huge physical urge to jump. It was terrifying. It felt like a split second decision that could have gone either way. No one could have seen that outcome at that point because I wasn't suicidal until that point, just very unwell. The reason I wanted to tell you say because people don't always plan suicide, sometimes it's something that happens in that terrible black fog of mental illness, at that particular point in time. So there may well have been no way to stop your DH, no signs that you missed because he wasn't in that place in until the crucial point.
I know you must know, objectively, that your 'friend' is jealous/narcissistic/a horrendous person. She just is wrong, she didn't know him in the way she thinks she did and she didn't know your relationship, she has no actual idea how ok he was or wasn't in the years before you. Only he knew. It sounds much more likely, to me, that you gave him something incredibly special, that as difficult as his life was he had beautiful years of love with you. People can live a very long time and never experience that. It's precious. It's not fair, what has happen to you. It's rubbish and unfair but it's not your fault and it's not his fault