Remember they didn't disclose the Proffer in case another Wagner wanted to plea, they didn't want them influenced by Jake's Proffer. With Angela they compared what she said against what Jake said in his Proffer and found out they were the same story. Comparing the stories weeds out the lies.
They may be waiting to see if Billy will come forward to plea and for the reasons I mentioned they want to keep the Proffer from him.
Why give it to George? George has made it clear he is not going to plea, he is going to trial. If George sees the Proffer of his mom and brother, this could be what pushes him into giving up his trial aspirations.
If George pleads then this will cause Billy to plead. If George doesn't plea then Billy won't. Billy will wait to see how George's trial goes I think.