So devastated to read this tragic update. Leo's little body has such a enormous challenge to recover from everything he had been dealt. Such tragedy for all of you. You really couldn't have done more. The fact he died on your watch was just the timing of his body giving up, nothing that lays any blame on yourself. It's just heartbreaking that you have all been through this last week to lose him anyway.
His circumstances remind me so much of Binky, though Binky's digestive system never started up after surgery. Another forum user, I think called Ruth (though I cannot for the life of me remember her user name) also had a bun with peritonitis at the same time, Harvey, I think, he had more invasive surgery than Binky and battled for longer. I felt so lost when it looked like Harvey would survive but so broken for his owner when he didn't.
However good and progressive veterinary care is, the bottom line is rabbits are fragile. When the dust settles, hold your head up high and know you did everything you could. I do not envy your emotional rollercoaster in the meantime. As my very dear vet said to me in the early days "you have to be tough if you have rabbits."