They absolutely can investigate evidence for the case and that is exactly what the text messages and contact between OSU, Purdue and Rutgers would fall under, evidence that this was common place in the B1G. Why do you think Michigan hired the #1 investigative law firm in the US? If there is a way for them to get their hands on evidence -- they will have the resources to get it done.
As the resident lawyer doggy said -- the case will come down to the facts -- if OSU, Purdue and Rutgers hands are clean -- they have nothing to worry about. IF they did it though -- they are guilty of the exact same 'integrity of competition' as Michigan. Meaning OSU, Purdue and Rutgers will be facing punishment.
Last thing you want to do in CFB is open a can of worms like this, as I don't care how "clean" you think your program is -- it isn't clean. No one is. These people are getting paid millions to win at a kid's game -- they are going to bend rules, look the other way and do whatever they can to get ahead.