I’m so sorry that you lost Snowball.
I think Buckeye is grieving, like you say you think yourself. I think you are doing all that can be done for him, you’ve given him a new friend, you see to it that he doesn’t harm himself by drinking too much, you’ve obtained advice from a vet. What’s most important now is that he keeps eating and pooping and moving about even if he won’t play or binky etc. I think having Einstein around will eventually help him a lot.
I think your buns will learn to trust you with time. Rabbits tend to need quite som time before they trust people. It’s probably because they are like the prey-animals of prey-animals and their instinct to run away is very strong. With my bunnies I have found that persistence and patience is key. You had to take Snowball away, of course, and from your description of their close bond it seems unlikely that you could’ve lured Buckeye away from her in order to take her when he didn’t see it. Leaving her dead body there just wasn’t an option.
When the boys become sexually mature, you should have them neutered at the earliest possible time, before they get into any serious fighting. If you do that, they are very likely to re-bond unproblematically after having healed from their neuters. Male intact rabbits fight even if they are friends – they can then “call it a truce”, be friendly and snuggly for a while, but they invariably start fighting again and they can really hurt each other, so you can’t let that happen.
I hope everything goes well for you and Buckeye and Einstein.