The story isn't straightforward, but these are my impressions re: your questions:
i. Yes. In the beginning, their relationship was very much based on her dependence on him. He truly loved her and that dynamic between them defined him. However, once she regained her vision she started to become more independent. He saw that his place in her world was shrinking and he was threatened by that. So he started diluting her drops with a mind to making her blind again.
ii. The actual scene made him uncomfortable because she was in charge...something he wasn't used to and didn't care for. I think he kept replaying the scene later on his tablet because he was meditating on the meaning of the interaction, trying to figure out if he liked this new relationship or not. Probably had mixed feelings about it. He seemed to trying to figure out her facial expressions.
iii. The words to the song... one that she composed... were about a relationship coming apart. She was looking right at him as she sang it, in effect, spelling out that she knew that they were coming undone and, by extension, that she knew of his role in sabotaging her recovery and getting rid of the dog.
For him, it was the final blow, the one that told him clearly they were done as a couple.
iv. From the letter that we hear via voiceover, James took the dog and tied it to a fence. He wasn't such a douchebag that he would deliberately kill the dog. He probably had a good idea that someone else would find the dog and adopt it. I was uncertain as to who found the dog and wrote the letter to her... it appeared to be someone who knew of her from the park, but not so close to her as to know her on a first name basis.
v. My wife and I discussed this and we both agreed that, yes, he did die at the end. Whether it was deliberate on his part or an accident wasn't clear to us.