I agree that my idea of realism could definitely vary from others. Maybe I've watched too many shows/movies where characters who are reluctant to talk about their crimes actually don't talk about it. Ozark is similar to the movies where they rely on a climatic court scene with a crafty lawyer who gets another person to confess to the crime and frees their client! Sure it could happen and probably does once in while, but I'm guessing it's pretty rare. Usually the guilty party shuts up.
About the Shaw/Javi interaction - Javi is refusing to supply Shaw because of the FBI raids on his trucks. She buys from another source. At this point, unless I missed it, Javi is still refusing to supply Shaw.
Shaw could 1) continue to buy from Darlene 2) start buying more expensive product from Madagascar again or 3) hope that Javi starts supplying again.
Shaw chooses to rat out the Byrds and cuts off the supply from Darlene. Javi is still refusing to supply Shaw. So by ratting out Darlene, Shaw is hoping that the Byrds can smooth things over with Javi and he'll start supplying Shaw again. She didn't even ask for that. Shaw just gave up Darlene and put all of her eggs in one volatile basket. She may hope that the Byrds can smooth things over and get the supply from Javi going again. He's already screwed her over by refusing to supply over an issue that Shaw has no control over - how does Shaw know that Javi won't do it again in the future? Seems like a bad business decision to choose a volatile, threatening Javi (currently not supplying) vs Darlene (already supplying). And if she's really intimidated, she has her own murderous psychopath she can send to eliminate Javi.
The show has characters like Shaw make dumb choices to advance the plot and create more puzzles for the Bryds to get out of. The whole Shaw supply was a series of dumb choices to create drama.