Just my two cents here. I agree with what you've said here Elle but, my thinking goes more toward MC trying to deflect to her brother and cousin because she is the one with the knowledge of how that scratch got etched into "her" van. Since the van was parked at Timmy and Chelsea's and cousin Joe was staying over there, the van was not parked the same way Chelsea left it and the keys were not in their usual place, that indicates to me that Joe took that van that night for a reason. The scratch was acquired on the van from the evening before to the next morning, the same night Haleigh disappeared. If the van was so far from where the Cummings MH was, how in the world would MC even know the scratch was there?.....unless she knew how it got there and was trying to implicate someone else that the "family cares about but, not so much". MC's words. Know what I mean? Since it was reported that Joe slept in his clothes that night, which was unusual according to Chelsea, the van had been moved, the keys not in the usual place, that tells me Joe went somewhere with the van that night after Timmy and Chelsea had gone to bed. Could be Joe went over to pick MC up so they could go on their drug run. Whether Tommy was a passenger or not, I don't know. Could be they were already gone on their run and when Tommy got to the house, it was dark and empty (a good reason he would be sitting on the porch "waiting").
I believe Haleigh could have been with them and the accident happened wherever that went down. Junior may have been asleep and never even realized he had gone for a joy ride. Seems far-fetched? I don't know but here is what I know about Oxycontin addiction. Oxycontin replaces a chemical in the brain that the body naturally produces. When abused, the body stops producing this chemical naturally thus the dependency becomes a raging beast in the user. Without the next fix, the person will do whatever they have to...to get it. Haleigh may have gotten out of the van while the deal was going down and they failed to check to see if she was back in the van. This is only one theory I've thought of and bear in mind, it's only my two cents. I just can't help going back to everything strange about that van and all the strange things that point to it in THAT 24 hour period.
The other possibility is RC was in this up to his ears and everyone else was a "gopher". He may have had a hard time keeping up a work schedule without his addiction being fed and something went awry between his last dose and the next and is either directly or indirectly responsible for what happened to his daughter. All the other events surrounding his actions could be just the inner workings of his drug world and part of how that was orchestrated that night. Whatever is closer to the truth, MC is protecting whoever it may be. She'll figure out eventually, spending her life in prison is not going to be worth it. In love or not. We just don't know when that revelation will hit her.