I'm very sorry you lost your brother.
I'll try and answer your questions from a personal point of view/experience as best I can.
Addicts lie, it's extremely unlikely he told you the truth regarding how clean he was for periods of time. Someone might tell family what they want family to know, not the reality of their use, or downplay their use, including telling others they are only using legal pills to provide cover for seeming high. Someone using does this because they don't want to stop using.
When someone is dedicated to their drug of choice they don't want what you would consider help. They want the privacy and space to carry on using.
Shifting blame to some girl giving him yet is just that, distracting from his own culpability. Addicts don't like to be blamed for anything.
Yes, anyone doing smack knows each time is a gamble, but it is not really a gamble in their heads at the time. It's a managed risk. Do a little test shot, short a tiny bump, see how strong it is. Sadly there are hot corners (badly unevenly mixed), or someone is just so desperate to get well they get sloppy, and then accidents happen.
As for why people start. Pain. Emotional pain. Depression. Loss of ability to see a future.
To a junkie help is another bag, not stopping ..it's not right, but it's how people think on junk.
To someone addicted to heroin, who does not want to stop, the last thing they want is to be forced or "helped" into giving up what they want to do
You might not have judged him for using, but the fear of others judgement is quite motivating in not letting them know someone is using. That and being forced to stop.
As sad and awful as it is, your poor brother didn't get to the point of wanting to be off the smack, enough to actually stay off it. There was absolutely nothing you could have done.
I promise.
I'm very sad he didn't make it through. If there is anything else I can do, or try help clarify, I'll try.