You state this so perfect. If I may add to it, JW wants his brother out. JW must testify against his brother to take the DP off the table. Jake will, IMO, downplay every action GW4 took that night. Jake will, IMO, downplay as much as he can get away with and still satisfy the prosecution.
Couple of examples:
IMO, Jake didn't know if prints were on the gun (etc), so he stated GW4 held the gun but wouldn't shoot.
IMO, GW4 wanted to protect Jake, not from Billy, but the Rhodens.
It makes no sense to me for Jake to place GW4 at the murder scene if GW4 wasn't there. IMO, Jake felt the prosecution knew GW4 was there and had to admit it to get the DP off the table.
I "think" the Wagner family has hopes that GW4 will somehow at least try to break the others out of prison. I "think" the Wagner family has discussed this during their meetings regarding what to do if any or all of them are arrested. I would love to know if they also watched 'Prison Break". I think the family wants one of them on the outside to take care of them and other things.