I wonder how much influence Earl Spencer had?
people.com/royals/prince-william-finally-reveals-who-decided-he-and-harry-should-walk-behind-dianas-coffin/
At the time, Diana’s brother, Charles, 9th Earl Spencer, argued against them being part of the procession — and some government advisers were wary too. But friends of the royal family have said that it was their grandfather, Prince Philip, who told the princes — then ages 15 and 12 — “If I do it, will you?”
William confirmed in the 2017 BBC documentary Diana, 7 Days, “It wasn’t an easy decision, and it was a collective family decision to do that. It was one of the hardest things I’ve ever done. But we were overwhelmed by how many people turned out, it was just incredible. There was that balance between duty and family and that’s what we had to do.”
Harry recalls, “I think it was a group decision. But before I knew it, I found myself with a suit on with a black tie and a white shirt, I think, and I was part of it. Genuinely, I don’t have an opinion on whether that was right or wrong. I am glad I was part of it. Looking back on it now, I am very glad I was part of it.”
...
The princes’ uncle, Charles, 9th Earl Spencer, called the walk a “tunnel of grief” and told PEOPLE and 2017’s The Story of Diana he didn’t believe that “tiny” Harry should have made the grueling walk.
“I was just so worried — what a trauma for a little chap to walk behind his mum’s body,” Spencer said. “It’s just awful. And, actually, I tried to stop that happening, to be honest.”
It may be no-one remembers who came up with the idea - I've had meetings like that - or everyone backing away subsequently and shifting blame.
I have to say I do find it hard to believe Prince Philip would have suggested it - I saw that clip of him and the Queen being told to "look after the boys" in a walkabout and how quickly and angrily he shot back