Snapedom

How does this relate to your last paragraph? The fact is, if Scumbledore had wanted to end the hostilities between Snape and Harry, he could have negotiated a truce at any time. He didn't. Instead, he fed their animosity because, as others have pointed out (including you, I think) he didn't want competition for Harry's loyalty. He was trying to create an unthinking suicide bomber, and he knew if Harry had the chance for a father-son relationship with a man close to his own age, particularly one who'd known his parents, the boy would find that much more desirable than a relationship with a man a full century his senior. That's why he got rid of Sirius, too.

Specifically regarding Severus, if you look at the chronology you've laid out, you've answered your own question. Dumbledore lied to Harry about Snape and encouraged them to hate each other for the first three years because he didn't want them to establish a friendly relationship. After third year he figured their relationship had been poisoned enough not to be a threat to him, so he could afford to back off a little.

However, he still needed them to be able to work together, so in fourth year he began to try to get Harry to mellow his attitude towards Snape a little. (He didn't have to manipulate Severus in this way because he knew he already had him bamboozled and under control.) That's where that "I trust Prof. Snape" blather comes from, as well as his other attempts to reassure Harry that Severus is trustworthy. Because he's a psychopath, he doesn't see how ridiculous that is. He just doesn't get that you can't spend three years poisoning Person A's mind against Person B, and then just snap your fingers and wipe away those years of hostility and suspicion that you've encouraged, and even set up!

That's what happened with the Occlumency lessons. Because Scumbledore saw Severus and Harry as objects he was manipulating (psychopathy), extensions of his own mind (narcissism), and not independent people with thoughts and feelings of their own, it made perfect sense to him to say, "Okay, I'm going to take these two people who hate and mistrust each other and force them to work together in the most intimate way possible [i.e., inside each other's minds], and it's all going to work out just fine because I say so!"

Then, when it didn't work out, Dumbledore blamed Snape without appearing to (because he really should have gotten over whatever-it-was years ago) and delivered one of his pseudo-profound and sensitive remarks about "some wounds are too deep for healing." That's true, but the wounds in question weren't the ones inside Severus he was referring to. They were the wounds between Harry and Severus he had created with his lies and manipulations.