What witness(es) would you think there are?
Are these any and all witnesses (like phone experts, forensic DNA specialists, etc.) or more like eye witnesses DH, etc?
All kinds of forensic specialists plus investigators. As
@Sister Golden Hair established, they had records from Ka-Bar that led to retailers in the PA region. They likely have evidence as to the knife purchase - so that would be a witness statement from the person leading that investigation.
If the ID of one of the victims was found in the car (is that what led to PA LE opening a broader investigation?), then I would expect some investigator would testify to that. There would be phone experts (and that would establish if there really were pictures of MM on the phone - the Grand Jury would be shown those pictures).
I don't think they had either roommate there, as I believe that's exactly what they were trying to avoid. They didn't need to be there, they were needed for the PCA.
Aside from forensic DNA, there could be forensic radiology and forensic footprint analysis. There could also be forensic hair analysis. The part of the investigation that focused on the car would likely have its lead investigator as a witness.
Someone would have to introduce the neighbors' audio and visual evidence. I do wonder how many cameras, ultimately, captured that Elantra around Moscow before, during and after the crime.
And I expect that are bombshells all over the place (including the autopsy records).
IMO.