No, you're not getting it. They can simply say that Yashiro pushed him off the roof. Period. No one else saw or heard the events at the roof after the celphone call got cut.

Your basically saying that it's going to be Satoru's word against his, but in this case, all the circumstantial evidence points to Yashiro. Satoru spoke to his friends and mother before he pulled off the plan. Yashiro took him to the roof and he fell. His allies even heard part of their conversation on the roof before Yashiro took his phone.

You simply have to imagine this case going to court. The prosecution is gonna massacre Yashiro. Satoru and the others have no reason to waste their time (and risk Satoru's life) to frame Yashiro, but them trying to trap Yashiro makes complete sense because of all the other factors.

I actually find Yashiro's plan of framing Satoru with the IV bag hilarious. The guy is on a wheelchair for crying out loud.