Can it be a spoiler when the series has ended and the new one isn't broadcasting in the US? Personally I think you can ask but perhaps we need a show of hands first!
For me I think the writers have been fairly unflinching in their portrayal of addiction. Deacon was obviously a serious health hazard 13 years ago - Rayna had to pull him out of hotel rooms not knowing if he was alive or dead. If she was on MN she would have had the usual addiction crowd (including me) reminding her of the 3 C's - she did not cause it, she cannot cure it, she cannot control it. She did the right thing in getting the hell out. (Questionable choice about concealing the paternity aside but there are plenty of threads here about the emotional damage done to the children of addicts). Deacon never really recovered; it was either good writing or chance that had Juliette refer to him as a dry drunk - a person who is managing not to drink but is not really recovering either. She should have been a bit more savvy about what this really meant given her history with her mum, but she couldn't help him either.
Obviously the whole 'let's have a massive row in a car' thing was just manufactured to allow the accident to happen, and Rayna was trying to stop him driving drunk as well. "Being a father might have sobered him up" - I do get this is a TV show but truly, honestly - this wouldn't have helped. No addict can get sober until they want to get sober. This bit is not Rayna's fault.
Gunnar and Scarlett - oh dear. Why on earth would he try to make up with her by offering her something that hasn't been a hell of a lot of use to her so far, namely a life with him? Cheap and selfish in my view.
In terms of the story arc for some of the characters, I wonder if the producers have been hamstrung by the mixed ratings the series has had? It may have been difficult to leave too much open-ended if the series might be cancelled, but of course that doesn't really explain the sheer number of characters who never get developed properly.
I love this show and I really want to see it find its stride in series 2.