Oh dear, I am so, so sorry.
I can tell how much you adored Kona, and just by reading your post I can tell you did everything
possible to help him and extend his life. In the end, you gave him the ultimate gift of ending his suffering. By reading your post, it doesn't sound like poor Kona was enjoying life much because of his health issues. As much as it hurts you, Kona doesn't feel anymore pain... he's on the Rainbow Bridge now, pain-free and happy...
I know the guilt you feel all too well. When I was 17, I had to put my family cat to sleep. My mom got her as a kitten when I was born. We grew up together and she felt more like a sibling than a cat. When she suddenly got ill, we took her to the vet and had them put her to sleep to end her suffering... we waited in the waiting room. This was in 2011 and I STILL feel guilt at times when I think about her. I regret leaving her alone to this day... the guilt lessens, but never truly leaves. When my dog died a year after, I held him as he passed and it was the best decision I've ever made. It was more peaceful than I expected, but still devastating to feel the life leave them.
I just wanted to offer a ton of hugs, tears and support for you. I know how absolutely horrible this feeling is... try to remember how lucky Kona was to have you as pet parents. You accepted all of his issues and did everything possible to help with his issues, even if that meant constantly being at the vet. I can promise you he knew how much he was loved and knew how much you helped him. Luckily for Kona he is pain free, healthy, and happy, waiting for you on Rainbow Bridge.
I looked at your pictures and Kona was SUCH a gorgeous boy. I'm so sorry for your loss.