The messages did show that the captain had issues getting enough sleep in the recent past, that he improvised poorly, that he "does not like to be in IMC", and tended to bank hard and lose a lot of altitude. From the investigation, I think we can safely conclude that he had sufficient rest at the time of the accident.
The FO did stand up to the him on one occasion, after one week he had her fly every leg so that he could get some sleep. On Thursday, she told him off, and on Friday, he gave her a "good job" plaque.
So, here's the problem. You've got an outfit with about 20 aircraft, 20 captains and 20 FOs stationed at 20 airfields. This is the FO's first job, and it's also clear from her texts that any action she took would likely result in her moving. Both she and the Captain seemed to have SOs in the area.
In late February, she texted this about talking to the chief pilot: "He asked me if [the captain’s] gotten any better and I'm like ummm..he can fly in VFR..lol"
So, like it or not, her SMS history paints a picture of her job and of the captain, that frankly explains how someone would not want to go around (and back to IMC), misjudge the approach on a downsloping tabletop runway early in the morning, bank for the final, and drop it hard.
If you expect a low-hours FO to stand up to a bush league captain, who is sixteen years her senior and two-and-a-half times her weight, well she did, just probably not in time in that dive for the field. Did she say something to the CP? Yes, just not necessarily clear enough that this guy was a danger to himself and others? Well, how much experience did she have? Normalization of deviance, etc. How much are you willing to say, "It's not so bad. He made it to age 47. Just another year of this and I'll have 1500 hours."?