Please cite a reference for this.
Oh, this Chimera stuff is possible but incredibly, incredibly, incredibly rare.
And if a perp was a Chimera, the two DNA samples would still be closely related; he'd be his own "brother", it wouldn't be COMPLETELY different DNA.
I'd put the chance that any of these prison releases due to DNA evidence being Chimeras being released as about zero.
Here's a description of the mild form. This is about the more severe form that I'm talking about. Here's another one. They should point you in the right direction.
By the way, I'm pro-life and don't think that this in any way conflicts with the idea that a child's life begins at conception, though it makes things a little more complicated to explain.