I think they will be continuing with political pressure, one possibility would be some new law that allows federal prosecution of nationals committing crimes, not just murder, against other nationals while out of the country.
One way around this would be to file a civil suit and force people to give depositions. I don't know about attorneys going this expensive route when there is no monetary compensation possible, all the expenses would be out of pocket. Getting a judgment that will never be paid is not realistic sometimes. I don't think anything less than a possible murder charge will make a difference, there would have to be a federal law for a lesser crime. Did the "friends" actually conspire to commit a crime while they were still in their home state? I think that would be hard to prove without some confessions.
This case reminds me of several others where there are known uncooperative witnesses, including "friends" that frustrate and thwart the families and LE from finding out what actually happened. At least there is a video of the assault that led to a chain of events resulting in SR's death. And, even if the results of the U.S. autopsy are not going to result in a prosecution, the family has more answers than some. For instance, in the case of Heidi Planck, a number of people apparently watched her die of a drug overdose and covered up her death by putting her in a garbage chute so that her body was taken to a landfill and never found. Despite her having been at a "party", none of the cowards will come forward and tell the truth.
It's confusing as to what Mexico's level of interest is in prosecuting the person that attacked SR in the video and the person that filmed the assault. The conflicting autopsies might cause a problem if they are prosecuted for murder.