I'm having some trouble making the connection between your theory that there were two perpetrators and your second conclusion (which I bolded above) that it then follows that the murders were planned and at least one girl specifically targeted. I would ask, can two people make a decision that if the right opportunity arose, they might try to abduct random children/teens for a criminal purpose? One example that sticks in my mind, because I read the victim's brother's account of the crime, is the murder of Jonathan Kushner in Florida. In this case there were two offenders who together decided that they would abduct and sexually torture a child. According to the confession, they planned to abduct a girl if the right situation came about but "settled" for a boy on that particular day because the opportunity was a good one (he had biked alone to a store to buy candy). The account of their decision was given in the victim's brother's book, "Alligator Candy."
I don't know whether this detail makes a difference to your theory - to me it seems significant - but the "forensics-related course at Purdue University" was not a regular college-level class where she would have attended with college students or even perhaps a college professor. I looked into this at the time and Purdue offered through their school of education a variety of science and engineering "experiences" or one-day workshops for middle school students, one of the popular ones was called Super Saturday and you could enroll your kid in a variety of science-themed programs. Due to COVID these have all been canceled for the current year so there is less information online now than there used to be, but IMO it looked like her "forensics class" would have been a one day or maybe weeklong drop-off day program that coincided with a school holiday like winter or spring break. It was a fun activity for science-minded young students but it wasn't a college course and she wasn't mingling on campus with the general Purdue student body IMO.
As you pointed out,
@Diddian, adults killing female teens for a financial motivation is quite rare when compared to other motives such as sexual assault or attempted sexual assault. If this was my theory I would collect as many examples of solved cases where teens were killed for known financial gain as I could and compare it to this one. Were the victims killed inside or outside of their home? Were they abducted first? Were they male or female? Were the victims involved in activities that had to do with drugs or sex work? And then I'd look at the Delphi case and see if any of the patterns fit.