I doubt the note was forged, otherwise the mother would've had to check his browsing history and done so before she called the police, which is ludicrous (unless it was produced in hindsight, but because the note mentions a remaining amount of SN in an envelope, and the police allegedly evacuated the house when they found the substance present in the attempt, I find it hard to believe there was any spare left to fake a note with).
His brother was there, too, I believe, not just the mother.
We would have to know if the message was typed or handwritten. It could even be that he left no notes but someone else wrote them by hand or printed them. If SS was on his screen or search history, and he'd been dead awhile, then there was opportunity to stage things. Doesn't mean that's what happened, but it is possible, and in light of what he wrote on the forum that was the direct opposite, I think it's reasonable to have such suspicions. I wonder if the police fingerprinted the envelope and papers, and if they tried to determine whether the prints were from natural movements. I also wonder if there is an agenda in which LE is involved and the note was faked afterwards and added to the report, as it's not secret that UK coroners have previously called for action against this site. I know this heads toward conspiracy theory, which has negative connotations for a negative reason, but SS is highly provocative and a political target in case you haven't noticed.
People will do things to protect their reputations or to come up with a believable explanation when they can't accept what is happening. When my boyfriend committed suicide with a firearm at home, his mother told everyone he did it because he had a brain clot. I don't think I need to go into a graphic explanation as to why she would have come to that conclusion. Maybe it was magical thinking, but decades later I realized there was a good chance he experienced abuse -- I didn't know him well enough at the time, I don't think I'd even told him yet about my own abusive situation, we were in the serious infatuation stage. But he was only 16, and super happy at the time with a lot of extremely positive things happening in his life, though it was discovered that he'd hidden the ammo months before after a hunting trip. Also, no notes were found, and as much as he and I prolifically wrote notes to each other (it was the eighties), I found it incomprehensible that he didn't leave me a note, and it's only at this moment that I wonder if he left notes that may have incriminated his family and were hidden; it's reasonable to suspect, but nor would I assume it to be true and outright accuse. That's the point of my whole comment here, to consider things reasonably rather than from a stance of pity, because it blocks considering all possibilities.
There was a movie, I'm intentionally not putting the title for personal reasons so I hope others will honor that, but in one scene a woman's family was going to be embarrassed by a story in the news. She freaked out and in the early morning, still in her nightgown, was picking up the newspapers from her neighbors' lawns to try to keep them from knowing.
People can do some amazing things under stress, sometimes cockamamie and sometimes very calculated, and people can go to extraordinary lengths to protect their reputations.
Like the recent story of the MP whose wife committed suicide after having visited this site or one like it (he didn't say it was SS but I don't know of another one similar). In the story, he both elevated and vilified his wife, and used some seriously slick emotional rhetoric, and my impression was that he was trying to cover something up by pointing the finger of blame elsewhere and trying to appear virtuous and dedicated to eradicating sites like this. People do that kind of stuff all the time. Either they try to cover things up, or they try to present themselves as heroic and on a righteous mission to deflect even the potential of scrutiny on them.
My own parents -- and my father is a retired homicide detective who absolutely detested lying -- permanently shut me out of the family when I demanded they be accountable for the abuse when I was growing up. Since 2018, my mother uses an old picture of us as her Facebook profile photo and talks for me in the comments as if we're still in contact. I know my father has to lie as well to support this story, there's no way he's not participating. That's utterly crazy, and easily disprovable if people would stop to wonder why, after almost a decade, I haven't come to visit and there have been no new photos of me. But protecting one's reputation can override rationality, let alone ethics, and people really want to believe.