Jodie I am very very sorry for all your losses and experiences.
I can relate in some ways, but it definitely doesn't change the pain you have been through.
My younger brother 6 years ago took his own life (he was 21 at the time). He was struggling with bipolar and alcohol, had some legal issues and I suspect possibly drugs were involved too (my parents won't talk about it). He got seriously drunk and used a .45 in the back yard of my parent's house. The odd thing was, we all expected a call like that one day, just not a suicide. He was the one like your brother, doing insane things in vehicles and such that we thought he was on a course to die young in some way.
My other brother who is a few years older knew some of the problems and tried to help him prior to the suicide. I didn't learn about this until later, and NONE of us blame him in any way. That brother is now in jail for heroine problems. I suspect some of his addiction issues are related to guilt over not being able to save his younger brother.
The sibling crash course definitely sucks. There always seems to be a tinge of resentment when I talk to my addict brother, the fact that I am fairly successful and have managed to fight through the mental history of our family to some degree is held against me.... yay.
I had a bit of a falling out before my younger brother died. I had been pretty rough about him in an email to my enabling mother, she forwarded it to him. I wish I could go back, talk to him, even if I couldn't save him, I want to tell him I love him one more time, but I can't, so all I can do is take a deep breath and move forward.
Jodie, I send you strength, because I know what its like to lose brothers to addiction, I couldn't imagine adding on top of that dealing with partners as well. You can get through all of this, lean on those that are experts, breath, and keep putting one foot in front of the other. I find practicing gratitude helpful, even if it seems out of place on some days.